In my case, rope arrows are basically the only things I need to buy. The rest of the items serve no purpose because I'm doing a pure ghost playthrough where combat and takedowns are impossible. That said, there are unlit candles placed throughout the city and the only way to light them is with a Fire Arrow. I assume that lighting them all makes something happen because there's a unique VO that plays whenever you light one. Unfortunately, the Fire Arrows cost 600 gold (with the 25% discount) so... yeah. We'll see if I can afford enough to light all the candles before I beat the game.