At least in the beginning of the game, the store only offers what you can find in the field, so it's not so much buying to advance, but buying to restock. And if you avoid battles, you don't need to restock, hence why it feels a bit pointless.
Hopefully that changes soon though, and then it would be more of the system you're describing.
Yeah, that how it is with me so far. Too much money, stickers album full... why bother with the puny enemies. Feel more like a nuisance than progression each non boss battle.