The obvious answer is because you like what you are reading, or looking at, but you can’t think of anything to comment about. You just want the blog owner to know you were there. Gives you a nice, warm fluffy feeling and it gives him or her a nice, warm fluffy feeling. Ah, bless, warm fluffy feelings all round.
Of course for the more focused, intense, driven blogger, there is another reason. You want the recipient, or any other visitors to the post you ‘liked’, to click on your ‘Gravatar’; your little picture or emblem that identifies you in the blogosphere (dreadful word, should have been taken out and shot).
But what happens when you click on a Gravatar? You get something like this: