What is D&D for?
No, seriously, if you can answer that (and I assume everyone posting here can offer their own answer - it's not meant to be a deep philosophical challenge) then you can answer the gold question. If you say that D&D is for X, then for you, gold represents success at getting X. It's as simple as that.
At its simplest level, D&D is about killing monsters and taking their stuff. You get XP for succeeding at the killing bit and GP for succeeding at the taking bit. You don't have to spend the gold, any more than you have to spend the XP. At this level, gold is not for spending, it's for getting. It's for keeping score.
Beyond that, it becomes a means to an end, not an end in itself. It becomes a commodity that is interchangeable with the means to attain other personal goals for the player (or, in the fiction, the PC). All the usual ones: power, sex, security, companionship, entertainment, and so on. There's no great mystery about it.
To see how important or not gold is in your game, imagine that every new PC starts with a million GP to spend on whatever they like, and they get an automatic million extra every time they level up. Normal prices still apply. Does that (a) ruin the game because there's no incentive to go adventuring or (b) make the game more fun because it removes all those irritating mundane constraints on having a good rip-roaring adventure? Ask around the table, don't prejudge. There's no right answer.