i use it to keep my dock clean, i hate it if there's a thousand icons down there. so now, instead of going into finder going through a lot of folders to find the application or document you want, you just use the shortcut that launches Quicksilver, (Command + Space in my case) and type in the first couple letters of the thing i want to find, then press enter and it launches. I was as confused as you are about it until i watched a podcast.
If you go into the iTMS and search for the video podcast MacBreak, you'll find a few episodes about it. They are very helpful.