Que: What is Page rank?
Ans: Page Rank is a numeric value that represents how important a page is on the web. Google figures that when one page links to another page, it is effectively casting a vote for the other page. The more votes that are cast for a page, the more important the page must be. Also, the importance of the page that is casting the vote determines how important the vote itself is. Google calculates a page's importance from the votes cast for it. How important each
To make a successful blog and boost up PageRank you need back links from other blogs and websites. The popular and simple way to get huge back links is posting of the comments in the blogs which have the DoFollow attribute.
There is all kind of people in the world (good and bad) which include the spammers who make abuse of this feature to quickly get the back links. Intentionally or for any other reason most blogs use the NoFollow attribute