Finding treasure is a core pillar of Dungeons and Dragons (alongside fighting monsters and exploring things). Treasure is the cheese that the game’s narrative structure uses to coax the character shaped mice through the maze of obstacles that the game is built on. Now there are other motivations that the players may have for how their characters move through their lives, but the game is built on the idea that people need treasure to function, and it provides a reasonable amount of it to grease the wheels and get the characters exploring the maze.
Low level characters scramble for enough coins to upgrade their starting gear, and things work out pretty well in that regard. Some things are hilariously priced (the 1,500 GP spyglass for example), where others are inexplicably available, like Potions of Healing, (which are the only magical items common enough to be found in the Player’s Handbook). Most characters can find enough equipment to spend their early adventuring rewards on to not have to worry about what to do with their money.
Once you start hitting the middling levels, (say around level 5 or 6), things start to go off the rails. Characters start to find escalating amounts of money and they actually require some magical items in order to be able to physically move their wealth around. Characters at this level have wealth, but not a lot of things to spend it on, which is a minor critique of the 5th edition system.
Previous editions and similar expressions of the D20 system have myriad options for characters to spend their money on specific magical items and other things like castles, fortifications, businesses, and a bunch of other things that characters would be interested in chasing down. 5th edition has some options for this (mostly found in the Downtime Revisited section of Xanathar’s Guide to Everything) but the system is built on characters having Downtime, which may not be something that the DM or the characters are using.
There are players (my brother being one of them) that want to turn dungeons and dragons into Fantasy Economy Simulator. They have this idea that if they apply business ideas and principles to the dungeons and dragons game, they can have their wealth work for them. They can build castles and fortifications and eventually become movers and shakers in the game world. They can become big shots.
Now i have no problems with this, but that sounds like an incredibly tedious way to play spreadsheets and statistics, er Dungeons and Dragons. There are a ton of different games that you can play with that focus (and honestly, the Ultimate Campaign book for Pathfinder takes a hard look at this type of a game). You can try to work this kind of a system into your Dungeons and Dragons game, but you’re going to have to work that square-ish peg into that round-ish hole.
So what do you do with your treasure as a character? Well, there are some options but this requires a DM that’s willing to work with you on some of it. You’re also going to have to decide what’s important to your character, and what they would want to do with their money. My personal philosophy for money in RPGs is “Spend every last GP, You can always go find more.” (Thanks Arn).
You can use the Downtime activities if your DM is willing to work with those optional rules to go looking for the magical items you think your character wants. The same rules also provide you with options for making your own magical items, and i think that’s a potential option for characters who want to be a little different. This puts you in unfamiliar ground because you and your DM are going to have to work on how the item works, what it does, how powerful it is, and answer a bunch of other questions. Other applications of Downtime that you may want to look at are options for building structures and maybe if you’re feeling it, vehicles. I hear houseboats are pretty awesome.
Other things you can do with your money, well you could spend it. Buy the tavern a round of drinks. Get shiny new clothes. Get some radical looking jewelry. You can spend your money trying to become a popular figure in the town your characters operate from. You can try and woo the local noble. You can give it to your character’s church. You can give it to random strangers. Money is your in universe currency to customize how your character looks (and potentially is perceived by other people) so spread some of that wealth around.
As a DM, you can invent a lot of things that the characters need to spend money on. Tax collectors, ambitious nobles, magistrates and a bunch of other folks have designs on the money that adventurers seem to find with regularity. I find that tying a real world need to a currency situation is a great goal to drive your players towards a plot hook, or if you sprinkle enough ideas around, they’ll go looking for adventures all on their own, to help out the people that they know.
Two examples of how you can use money to help drive your players towards an adventure:
The characters (including a cleric of the Forge God) are called to the Temple of the Forge God in the middle of the night. The Temple has been good to the characters and has been there whenever they have had serious needs. The high priest is distraught because one of his underlings has stolen all the money from the church coffers and disappeared into the wilds. He or she hates to do it, but asks the characters for a short term loan to keep the church afloat, at least until they can get some help from folks higher up the Church hierarchy. The amount of money the church needs is a substantial drain on the characters funds, but without the church is ruined.
So the characters have a couple of choices, but i think most would help the church out. That opens up more choices. The characters need an infusion of cash to keep their own ventures operating, so they are going to need an adventure of their own. They can try and chase down the missing priest (and recover the stolen cash), they can look for other lucrative opportunities for adventures, or they can try and find more help for the church in other places.
The characters have just rolled back into town and are looking to kick back at their favorite tavern. Unfortunately, it’s been closed and the property seized by order of the local nobility. Something about unpaid taxes, sedition, and a host of other legal troubles have left the place closed and the family that runs the place ruined. In order to open the place back up, the characters are going to have clear the debts and hire a lawyer to handle the legal issues. That isn’t going to be cheap.
Money and treasure in an RPG are resources that characters can use for a bunch of different things. Characters can use them to customize their equipment, their personal spaces or on personal luxuries. DMs can use the need for money to spur adventurers forward into areas and to chase down leads.
TL:DR: You’re the Mice, Treasure is the cheese, the adventure is the maze. What you do with the cheese is up to you, but you’re going to probably need more cheese.