Lot of spammers may use the comment section of a blog to post spam messages and links that promote their websites.
Adding rel='nofollow' attribute to link will intimate the search engine that the particular link is
not audited by your site, and should not be counted as a trusted vote for the popularity of the link.

Although this definitely do not prevent spam, but it remove the motivation that results in
a spammer target your site. The provided link will work, but will not be as desirable to a spammer
as it offers a diminished link equity value.

You can also use the same technique when including links to sites that you don’t want to “vote.” Here’s an
example:

Spam site!

However, too many links without rel='nofollow' may hamper your rankings if they are linking to 'bad neighborhoods' as well as damage your reputation and credibility.

Currently, all major search engines support nofollow feature.