I have written on this topic before, but it fits well here so let's take another look at it. Blog commenting is a win/win for both parties.
For you, it is a way to connect with other bloggers as well as getting that all important backlink. The other person benefits from having valuable comments on their blog.
Blogs with lots of comments tell a new visitor two things: a lot of other people are reading this, so maybe they should too; and if many others find reason to comment, it must be good material.
Many bloggers, especially the up and coming ones, will return the favor and comment on your blog. This makes both of your backlinks reciprocal, so they do not count as highly as a one way link, but the benefit of having those comments balances the equation.
This is the last post in this series. Tomorrow we will revisit Backlinking method #1, Article marketing with a series of posts on how to write articles quickly for backlinking purposes.