For example, recently my choir's recruitment co-ordinator was on holiday during a week when we knew that some people would contact us to ask about membership. By giving me edit-access to the audition-scheduling spreadsheet, I could look after these messages for her, and she had immediate access to the up-to-date information when she came home.
Giving everyone in the whole world (well at least everyone with a Google account) access is very easy: you just change the first, generic, option in the access control lists.
But giving access to specific individual people is a little more desirable and only a tiny bit harder to set up.
