Well 4 of them are drills - very convenient for pilot/main/countersink etc, plus an angled drill. Some of them are also garden tools, like an excellent pole chainsaw for lopping. Others are cordless versions of corded tools I already own, eg router, jigsaw etc. I love my corded Makita jigsaw, but have very rarely used it since I got the cordless Ryobi. Just recently I got a cordless, high power light, which blows away all my lead lights. And, as has already been mentioned, their cordless nailers are brilliant.
I thought I made my reasons clear - this way I don't have loads of different battery chargers/batteries, and I can save money on bare tools. It works for me, and I don't feel particularly 'locked in'. A tool would just have to be significantly better quality to persuade me to get another set of batteries and charger, as was the case with my Festool cordless sanders.
I've also bought bare tools that were cheap enough that I could try them out without worrying too much about lack of use, or they came on promotion with batteries that effectively makes the tool price close to zero. Hope that helps - like I said, it works for me.