The point is for increased productivity.
Thunderbird on the right, Firefox on the left - so when I click a link in Thunderbird instead of hiding it (by bringing Fx to the front) I can still continue reading other email in Tb, and I can also see the link in Fx as it opens up.
RegeditX on the left, xplorer^2 on the right - allows me to sift through the registry and find the actual file associated with any given key without having to ALT-TAB back and forth. Picasa Web Albums - johnlgalt
(this one was too big to upload here, @ a whopping 4.44 MB)
Play a game and watch a TV / Video / Movie / Sports / etc....