Why do "trailers" come *before* the movie?

I saw "Quantum of Solace" this weekend, movie 2 in the James Bond reboot. Was it the best Bond movie ever? I don't know about that, but it was pretty good. The action of Bourne, with the convoluted plot of a typical Bond romp. A little light on the gadgets for my taste, and totally Q-less.

Speaking of reboots, the trailer for "Star Trek", J. Abrams reboot of the series of the same name, premiered before it. I also have mixed feelings on this one, though it looks to be a fun special-effects extravaganza. I like that they kept the bright color pallet of TOS, sound effects, and other ques, but there are too many explosions and Vulcan emotions (and that's just counting the trailer).

Watchmen also released their full trailer. I'm really excited about this one, in no small part because Zack Snyder really knows how to cut a trailer. Both Watchmen trailers are fantastic, and integrate music in a way most editors don't even attempt.

Harry Potter also doesn't want to be left out of this trailer-fest.