Been trying to answer that question for a while. Former CNN anchor Rick Sanchez explains it pretty well:
It enables journalists to send news updates with the immediacy and ease of Twitter. With no 140 character cap, spelling, punctuation and key words and sentences don’t have to be sacrificed. Posts can be short in length but long on substance. And posts can include pictures and video, without having to click on a link or leave the site — information and images together. Journalism is at its best when it provides detail and context.
If you follow someone on Tumblr, then you get his or her updates instantly, you can read them in your dashboard feed and “re-blog” the content you like, all of which makes Tumblr that hybrid blog and social network content stream.
OK, maybe it’s time to get a Tumblr account.