Crying relaxes all the tension from the body, which is why it is good for you. Then you are relaxed enough for sleep.
Many people who cry save themselves from a lot of built up tension, agression and repression and may avoid a more serious depression by doing so.
Crying can act as a release sometimes when you're over stressed and actually helps clear your head a bit. So its a good thing to have a cry every now and then (but not all the time).
Captain Swab, what the hell is that all about??? Did you not read the question? What were you trying to achieve with an answer like that?
Waaii and Jump are absolutely right. If you can have a bit of a nap after a good cry when you are upset, you will feel so much better afterwards. Hope you're ok Bexxie. :o)