In addition to the things Richard mentioned, if you are doing a lot of applications, it's handy to know where you sent what, when, so that when companies get back to you, you can easily pull up the info, and know what position they're referring to, etc. As some companies' hiring timelines can be rather long from initial ad to interview contact, if you're sending out many cvs/resumes/applications, some will slip your mind as time goes on. Keeping track of what, where, why, to whom, and when can help refresh your memory.
As a long-time job seeker who finally gave up and started her own company, I would somewhat disagree with #7 in Richard's list (as a personal aside, this only applies to me). Looking over my records got a bit depressing in the end, so I quit unless I received a response. There's only so many 'Nos' one can look at and still draw motivation from. But, YMMV of course, so if it works for you, great. I guess one could say that the 'Nos' motivated me to strike out on my own, so they were helpful enough in their own way. Not optimal, but whatever works, I guess. :D