I sign email for the same reason I say "hello" to people when I see them on the street--doing so fulfills social expectations regarding kindness and politeness, making communication seem more personal.

If you sign your email "Thanks, Brian" or "Regards, Brian" or "Cheers, Brian" then that is much better than "- Brian" or nothing at all. Similarly, I include "Dear XXX," or "Hi XXXX, " or at least "XXXX," at the beginning of the message.