For the blogger it's great as
1. They know I have actually read their blog
2. They get a little feedback, even if it's just a one liner!
Its good for the commenter if they have their own website as there is usually a separate box to add this.
It however fustrating when they don't approve it. I have had one blogger email me, thanking me for my comment and asking if they could have a freebie to review, but didn't actual remember to approve my comment. I consider this an own goal.
When I make a comment I make sure that my comment
1. Is relevant and refers to the blog.
2. Does NOT include a website address. There is no need to put it into the comment, as this will label the comment as SPAM. By adding your website address into the website address box and nowhere else it is then embedded into the name you use when you post your comment
It is pointless making a comment which will be treated as SPAM, as it is unlikely to be approved. It does put bloggers off and many remove the comment options because of this.
Approved comments are a "Win Win" for both the blog site, and the commenter site.
Do you understand how to make a "Win Win" comment!