Ever try to read an article, blog post, comment, etc. that consists of one huge paragraph with no breaks? Did your eyes glaze over? And, did you decide that you were simply not that interested in reading it so you moved on?
This is what I refer to as the impenetrable wall of text. It doesn't matter how interesting the subject is - if it's difficult to read, most people will give up.
This becomes a particular issue online because of issues like eye strain caused by staring at a computer monitor all day and the ubiquity of hand held devices with small screens.
Like it or not, people tend to scan, not read. Make it easy for them to pick out the important things you want to say by breaking your text up into shorter paragraphs. White space on the page gives the reader a chance to pause and their eyes a chance to rest. Which means they may actually read what you have written.