Why would you choose Alfred over Spotlight?
One thing is speed. I believe Alfred is currently faster than Spotlight. And, this is specifically for Lion I believe, Alfred learns where Spotlight doesn't.

If I want to launch Adobe Photoshop and start to type 'Ph' Alfred will offer Photoshop as an option and move it to the top with the more I select it.

With Spotlight, Photo Booth with be the first choice, and it will not learn what I want with repeated use. I believe at one point it did, but lost the ability in Lion.