Digital BW, The Print

At the risk of displaying my ignorance, can someone explain (or point me to 
an explanation) of the benefits of using a tablet over using a trackball 
(which is what I now use with Photoshop) when it comes to image work?  I've 
been to the Wacom site, but it seems to assume you know what a tablet does, 
and only talks about the differences between their various models.

People who use them seem to love them (judging by the posts here) and they 
aren't all that expensive for a small one, but I'd be buying one blind at 
the moment.  So any advice is appreciated.

If this is getting too far off topic, please contact me directly rather 
than through the list.  Thanks in advance.

Cheers,
Kip