If you have to ask why you would want a smart phone, you likely do not need a smart phone. Most smartphones are rather weak as phones. If all I was doing is talk, I would prefer a clam shell like the RAZR.
As for what to do with a smart phone, it depends on your interests. I use mine as a GPS, check the weather, listen to music, watch a video, and countless other things. I do not use the phone part much.
As for cost for data, Speakout is close to the cheapest out there, $10 a month.