There is a public bathroom that anyone who wants to use it can. However, when someone is inside, no one else should use it. There is a single key to open its door, and the holder is the only person who can use it.
When two streets cross, there might be a stoplight in the corner to organize who can go at any time.
A lock limits who can use a public resource, in order to stop many people running into each other when trying to use it.