As ShtBrix already mentioned it is for a professional looking paint job. If you do not want to use the sandpaper you do not have to. However, I would recommend at least some sanding to smooth down the primer prior to applying the paint.
i'm doing a matte color and then a clear coat of acrylic.
Matte black, want to get the interior to look just like the exterior which is black.
I'm not sanding the paint. Just the metal to get it rough so the primer sticks, and then I sand the primer a little and put another coat of primer on it,and then get a finer grit and sand the primer more to get it smooth and then i begin to paint.
Something like that.
Off topic: Heh I gotta change my alias on here sense I chose to get the HAF 932 case instead of the Antec 1200.
Which I doubt that's possible so I gotta make a new account lol.