The functionality is like a library book. If you have it checked out its exclusively yours but it still belongs to the library and can only be viewed or checked out AFTER it has been checked back in.

This function is most usefully when the project part of a larger database or a centrally housed database.

the benefit of checking it out is that you have exclusive use of it and no one else can make any changes to it. Another benefit is that if you check it out you can work on it locally on your on machine this is very useful if you are somewhere you do not have a connection to the database and it should be faster working locally. 

When you have finished working on it you can check it back in and everyone can have access to it again.

lets say you had a long flight from the east coast to the west coast and wanted to work on the schedule in the airport and on the plane. You could check out the project at the east coast office work on it while traveling (LOCALLY on your laptop) then when you got to the west cost office of somewhere with a fast internet you could check the project back in.

so basically what you are doing is temporarily taking the project out of the database to work on it then bringing it back into the database when you are done working on it.