Because we can! Just tweet away, reply to other tweeps, you will find that if you tweet something interesting, tweeps will re-tweet your tweet and possibly reply, too. I have made a lot of friends since I've been on Twitter. If you have an interest, tell twitter and then follow it.
'Cause it's curious to put your impression into limited number of signs. Interesting to make your limits more creative))
If nobody replies to you tweets on twitter, possibly it's because they aren't that interesting? haha. Only joking