We all have come across long article and comments that go on endlessly without a paragraph break.
Postings like that are difficult to read, particularly on a screen, because if you take your eye off what your reading, even for a millisecond, relocating the place where you were reading, when the text is just one big block of words is often difficult, and can cause the reader to lose their train of thought, or worse, lose interest in reading the article.
Paragraph breaks give a reader an instant to pause and absorb what they just read before moving on.
Here's what Christopher Merrill of Common Sense Web Design has to say on on the subject;
It’s a simple thing, but it’s too often ignored by the new web site owner:inserting paragraph breaks to organize thoughts will make your text easier to read.
Avoid posting long stretches of unbroken text. Users will shy away from reading your material; the search engines will pay less attention to you as well.
Instead, insert (bold) headers that will draw the right audience, and attract the search engines. Regular paragraph breaks will help you to organize your material too.
And here’s an added benefit in using (bold) header tags and paragraph tags: breaking up your text will make it easier for users to relocate material. And when users come back — the search engines can detect the repeat visits.
Inviting someone to your website should be like inviting someone to your home: you want to make them comfortable, so they will come back again.