In any multi-user environment you must deal with cases of simultaneous access. On file servers, you lock a file while you are using it. You unlock it when you are finished with the operation. The other user must wait for you to finish.
If you do not lock files, the data can be corrupted. A simple example is with two people editing a document. If both are editing at the same time, then one saves his changes before the other. The problem is that the first set of changes are over written by the second.