When blogs first started, they do have do follow link on them. It was view as a way for Google indexing spider to crawl and discover sites. It doesn’t take long before spammers abuse it by commenting on blogs comment. Spammers spam comments with their own site links, trying to promote all kinds of products, services sites.

To counter this, Google implemented the "no follow" tag. Major blogging platform pick up this and implement for URLs in comments as standard in their scripts. What it means is that URLs insert in comment field is ignored by Google. No Pagerank or indexing benefits are passed on for the target sites. The expectation was that this would dissuade spammers from commenting on blogs.

Unfortunately, the "no follow" tag did have one unforeseen consequence. Folks that would place legitimate comments on blogs were also dissuaded from doing this because they didn't get any "link juice" from commenting, either.

By setting your blog as a "DO follow" blog, you are encouraging others to comments – both from genuine commentator and spammers alike.

Most major blogging platform has built in comments feature to moderating comments that would retain comments made to await approval from blog administrator.

This would put in some load to blog owner and if you’re not ready for that you might think twice before allowing it.