File Corruption:

You know a Photoshop file is corrupted when it won't open.  It has happened here in the Mac Lab where Photoshop files have gotten corrupted and students have lost many days of work. 

Protection From Losing Work

If you save a new copy of your file for each day of the week, you will minimize your loss of work to just one day in the event of a Photoshop file getting corrupted.