Just imagine a better Jerry Seinfeld impression than I can normally do when I ask that.
Seriously, there are so many forms of online self-expression available today--Facebook, MySpace, Orkut, Buzz, LinkedIn, Twitter, email, message boards, any website's "Comments" section, IRC, IM, SMS, Foursqaure, Digg, ePinions, Yelp, Flikr, YouTube, Skype, ChatRoulette or even standing in the middle of Stormwind City and informing the whole World how you feel about people of various races and sexual orientations--that one must wonder what to do with them all (besides 'baleet with extreme prejudice').
Now I'm not going to start debating the possibility of information over-saturation, but I will say that I don't feel like it's necessary for me personally to broadcast my thoughts across the Internet unless they can either a) make someone laugh or b) teach them something they didn't already know. Both are things I always appreciate when done for me and try to do for others.
As for the title question, I guess the necessity of a blog in my personal case is to have a bit more room than 140 characters or whatever to talk about a given subject. This will usually be lessons learned and experiences I had working on a project, rants on life's various annoyances (many of which seem to be manufactured by Microsoft) and general commentary on life's colorful array of bullshit.
I know this entry has not been terribly funny or informative, but it is just an introduction so if you don't like it you can kindly pack your bags and take your whole family on a nice trip to Gofuckyourself (it's in the Bitemenesia Islands). But I don't want you to go there empty handed so I will provide you with one of the two main purposes of the Internet. And since I don't have any porn to post, here is a picture of a kitten: