@Captbnut
Here are some answers! They may not apply to games run by someone else:
In my campaigns I play it less gamey and more roleplay. Thus, you can almost always find somewhere to spend your money, just like in an actual city or town. Indeed you can probably buy the clothes from someone’s back if you’ve got enough money, or are persuasive enough. In a large city finding a vendor for most items is trivial, in a smaller town specialty goods are rare or impossible to get. For special magic items or superior craftsmanship, you will need to find specific NPCs who deal in such things.
Treasure is a classic trope of old school RPGs, and Dungeons and Dragons is no different. I try to give out gold relatively liberally in 5e, as I like making custom items, and I want players to purchase them. While the greater motivator for your characters in this case is Shikka’s esteem and advancement within her court, a reward is certainly awaiting your characters at mission end. You can expect monetary or items of monetary value as a reward for most quests, though I may not have an NPC specifically hand them to your characters. If you are curious about the specific reward from Shikka, you may ask her!
It does depend on what kind of campaign someone is running. If we were going to do perpetual dungeon crawls, then spending as much as possible for every available advantage would be advised. For something more story heavy, some flexibility with money is a good idea.
Well, even in my campaign - if it ever seems like something life or death is imminent, I suggest spending all your money on advantageous tools/weapons/allies.