When you need to remove your faucet aerator to clean debris out that is restricting water flow, a key comes in very handy.
I could not get the aerator out of the spray faucet without this tool, which likely came with my faucet when we installed it. But after two years this is the type of thing you may forget you have.
The key was in the bottom of my plumbing tool basket. Luckily I managed to find it and recognize it as the tool I needed.
Happy plumbing!