4passive damage that ignores walls
Equip it and clouds of spores drift off on their own, no button press involved. They travel through solid tiles, so anything on the far side of a wall still takes hits. The damage is classless, which means none of your armor or accessory bonuses touch it, but it also means every build gets exactly the same value out of the slot.
it needs somewhere to grow
Spores only form close to solid tiles, or on background walls, and the wall version reaches much further out from you. In an open sky arena you get almost nothing. Inside the Dungeon or the Underground Jungle, where walls are everywhere, the field around you stays full. If you are stuck somewhere bare, dropping a handful of blocks nearby restarts the flow, and temporary ice from an Ice Rod counts for this too.
when to actually wear it
The best use is anything where you are not aiming: fishing, digging a hellevator, waiting on a farm. It also quietly solves Dungeon casters, since the spores are not the kind of hit that sets off their blink, so they die where they stand instead of hopping around the room. In a real boss fight the output is small enough that a class accessory usually wins the slot.