Why should I use Quicksilver instead of Spotlight?

Spotlight can luanch apps, just like quicksilver can.

What makes quicksilver is because it is more of a shell. And can be extended.

Because of those two things, it becomes more like an interactive spotlight. It can not only return what you are searching, but can actually perform actions on what you are searching. Either e-mail a file, move or delete it, search the content of a file.

I believe you can even interact with web services.

I would suggest reading more on it, if you can get use it, it can provide a lot of shortcuts and save some time http://qsapp.com/about.php

If not mistaken, spotlight is more to file search. Quicksilver is like a quicklaunch for your app. I use it extensively as it is simple to type in the app name and launch the app. It's free, no harm to give a try. cheers