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. There are a couple of locations in the starting areas where you can use your key to access something fancy. I'll run through how to locate them and find your way to each.
Where to use the Simple Key in Silksong
There are two different locations where you can use the Simple Key in the lower reaches of Pharloom.
Directly above Bone Bottom
You can reach this door once you acquire the Swift Step dash ability in the Deep Docks. To get here:
Travel to The Marrow Bellway and head left through MosshomeDrop down at the far end past the pilgrim enemiesHead immediately left to find yourself above Bone BottomJump up to the edge of the chasm and use the midair dash to reach the hanging cage, before dashing again to reach the other sideGo through the entrance and hide under a platform from the acid-spitting bug, then sprint to the far end to avoid itJust in the next room, you'll find the locked door on the left. This leads to the Wormways region.
At the far side of the Deep Docks
This is just after the mini-boss with the ringing bell and all the forge enemies. To get here:
Travel to the Deep Docks BellwayHead right across Deep Docks and hit the lever to drop down instead of carrying on to LaceTake the lifts down and go right to find the miniboss arena.Continue past the arena and over the lava to find the doorHonestly, I don't know where this door leads because I used my one key from Pebb on the one above, but I'll update this when I find another.