Why should I keep multiple versions of the same file?

Professional graphic designers keep multiple versions of their files in order to keep track of client-requested changes, which are billable. It's the equivalent of taking a "snapshot" of a project at a specific state.

Students should also make it a habit to keep multiple versions of their projects for safety reasons. If the only copy of your project gets damaged/corrupted/lost, you will have lost hours of work (maybe weeks). "File > Save As" is very useful because you may want to revert to a previously saved version from another day. Some applications also create cleaner and more compact versions when you choose "Save As." We recommend naming your files with a number after the name so that you can track different versions (i.e. "My File-2.doc" "My File-3.doc" etc.)

Bonus: if things go badly, you can always start again with the previously saved version.

Hint: if your personal copy of an application at home does not match the version in the studio, you may need to keep multiple versions. Some applications will allow you to save to an older version that can be opened at home. This function might be located under file > export or labeled "legacy." (Also see Saving Backwards)