There are different locations to use this key in the starting areas.
Finding a place to use the Simple Key in Hollow Knight: Silksong is one of the early item mysteries you’ll have to unravel. Considering it’ll set you back 500 Rosaries (a lot of currency at the start of the game) if you buy it from Pebb in Bone Bottom, you’d expect the secret it unlocks to be equally significant.
Usually, items like this unlock hidden areas or even give you early access to late-game regions—this is a Metroidvania after all.
