Let's say you have just spent few weeks in some exotic country with a bunch of friends. Everyone had a digital camera and made thousands of photos. Now you have several directories with hordes of oddly named photos and you just want see all of them in the order they were taken. Throwing them all in one folder and setting file ordering by date might help but everyone's camera can have a different internal time. File system dates can also be lost on their way to you (FTP upload etc.).
Being a programmer, rather that searching for some program on the Internet, I wrote my own quick and dirty command line tool for this task (mostly hard-coded paths etc.) in mid 2010. This year, I added GUI (where all the settings can be adjusted), some more date & time functions, and basically made the whole thing usable. And finally, beta release of PhotoMixer is available.