I didn’t answer because you already gave plenty of examples of what it can be used for. You don’t need me to answer...you already know.
The "you" in my post was being used in the impersonal sense (ie
one doesn't answer the question of what money is for in the context of gameplay by producing a mediaeval price guide, which in any event the game includes.) A price list isn't advice on gameplay.
You are correct that I can import advice on gameplay from other games into 5e - but that's not a super-strong defence of 5e's design, I don't think, especially as some advice will probably contradict how 5e is meant to play.
There are no charts or guides for many of the things that wealth can be used for in the context of the game, but that doesn’t mean it can’t be used. I can give you plenty of clntext from my own game....but would that mean anything to you?
This doesn't speak to my point, which is
what is the gameplay purpose of imagining my PC spending money on those things?
If the answer is
just because it's fun to imagine it - ie the expenditure is colour and nothing more - then maybe the book could come out and say so:
the goal of playing a game in which you must succeed at wargame-like challenges to have your PC collect gold from a dungeon is to then imagine your PC spending that gold on whatever you want to.
D&D 5e also answers it by giving you the price to upkeep a keep or castle. Clearly you have to spend money to buy or build one before you will be upkeeping it, or maybe you inherited or married into it. However you get there, it's an answer that 5e gives to you. It just doesn't give you the build prices. The DM can hand those out, though.
As I've already said, in this post and an earlier one, a price list is not advice on gameplay.
Why do I need the game to tell me what treasure is for? I can decide or myself whether I want to give it away, hoard it, use it to build a castle and land, purchase a title of nobility, use it to influence the world in some manner, collect it in order to try and persuade someone with a magic item I want to sell it to me, buy jewelry for a significant other, and on and on and on. I tell the DM what I am doing, and the DM sets the price and difficulty numbers if necessary.
Clearly treasure is for spending!!
But what is the gameplay purpose of that expenditure? It's not unclear in classic D&D (building a stronghold attracts followers and allows an army for engaging in military campaigning, which was an assumed part of the game back in the 70s). It's not unclear in Classic Traveller (buying a starship let's you engage the intersteller travel system, which is a core part of Traveller gameplay). It's not unclear in 3E or 4e (buying magic items expands the list of character attributes and capabilities).
My own view is that if a game is going to make an in-fiction goal, like acquiring treasure, an assumed focus of play, and is going to treat that in somewhat tedious detail (keeping track of all those gold pieces, having all those detailed price lists, etc), then it might address the question of
why?
See here is the rub, in 5e or any game system, the impact of "fine clothes" should be very situational and setting dependent. So, how many tables of rules chart of clothes vs situation vs gp cost do you want.
Well so should the effects of praying to a god, or the costs of buying a sword or a suit of mail, but 5e is happy to get pretty precise about those things.
I'm not sure why it's OK to tell me that a + such-and-such-amount to my damage pool, or my defence number, costs this much gp; why being able to repel a vampire with a prayer requires such-and-such details of PC building; but doing something of this sort for social skills is out of bounds.
in my experience, once you take something as mundane as clothing choices and distill them down to "other ways to get a plus" you typically wind up heading towards the Christmas tree of pluses.
In my experience, if making a move in the game ("I spend 100 gp buying a nice shirt") doesn't generate consequences for action resolution mechanics, then players can't make that sort of move as part of engaging the game and its fiction so as to change it.
D&D has never taken the view that "I carry a longsword rather than a dagger" is mere colour (at least since OD&D, and even back then I think that daggers might have been an exception to the d6-for-damage rule, although maybe I'm getting confused with EPT). It's never taken that view about "I wear plate and mail rather than just a loincloth". But presumably you don't think it's tumbled down the slippery slope you're worried about as a result.
I'm not sure why rules that relate finery to certain social interactions would cause the problem when the combat gear rules don't. I can't say I've noticed it, or any hint of it, in my Prince Valiant game. (And more complex games can use other devices also to manage this - eg Burning Wheel has an advancement system which means that a player has a reason not to always want to use as big a dice pool as s/he might want to.)
My character plans to spend gold helping children in towns we pass thru, setting up allies and contacts in places we leave, staking businesses and using glyphs and sending to help establish a network. Silly me for forgetting that "what treasure is for" is really "another +1 somewhere." The rules for this... in the book.
So it's powergaming to improve social skills, but it's not to just ask the GM for favours? Or arw those allies and glyphs and whatnot just for colour?
But putting that to one side, you can write what you're describing into a rulebook as well: eg "You can spend the money your PC has taken out of the dungeon on various social projects, which your GM might then have regard to in setting the DC for interactions with the NPCs who benefit from those projects, or perhaps in deciding that no check is required to have them help you."