I never really used categories on my blog until one day I had an SEOpiphany. Since it's not like I have a ton of inbound links to this site I figured I needed ways to make my own internal ones. Enter blog categories. I get a lot of traffic for the music of Grey's Anatomy so I thought "Hey, wouldn't it be great if each time I posted a Grey's Anatomy Music entry it would give me a link back to myself." So now I have the categories but have yet to see any seo benefit. Time will tell I guess.