When it comes to leaving comments and backlinks at blogs and web sites not all blogs and sites are created equal. In case you never heard of it every web site is assigned what is called page rank.
Page rank is like a grade given to web sites by search engines. Page rank can be anywhere between 0 - 10. It is a numerical value assigned to a web site that indicates the quality and size of that web site.
Very big, well known, authority web sites like Ebay, Amazon and Yahoo have very high page ranks of 7 or more. Brand new web sites have a page rank of 0. Most web sites have a page rank of between 1 - 4. You can see what the page rank for any site is at whatispedia page rank.
Part of what determines a sites page rank is the number AND quality of backlinks pointing to it. Links from sites with a high page rank have more power and value than a link from a site or blog with a low page rank.
So when you leave a comments at a blogs with a link back to your web site you are adding to the factors that will result in an increase of page rank of YOUR web site UNLESS that blog assigns the rel="nofollow" attribute to links left by blog commenter. Nofollow is a blog, essentially, telling the search engines:
Now the first conclusion most people would make is:
In theory you are correct in that assumption except for one thing. Search engines do not like it when someone does something to purposely manipulate search engine results. Search engines value what real web site visitors do because, after all, search engine are built for normal people. If a normal person visits a blog or site and leaves a comment with a link to a web site it is usually because that person has had a good experience with the web site and they are sharing. This is, basically, a real person giving a vote to the site they are linking to.
You or anyone going to web sites and blogs leaving comments and backlinks for the purpose of gaining traffic and increasing the page rank of your own web site IS purposely trying to manipulate the system. If you concentrate all of your backlink building by leaving comments at only sites blogs with high page rank that do not assign the nofollow attribute this linking profile will be detected by the search engines as being artificial and can actually HURT your blog commenting and backlinking efforts and sites page rank.
To avoid this we want our commenting and backlinking to LOOK like its coming from real, regular, internet surfers. The way we do this is by:
Your comments and backlinks to nofollow and low page rank sites is NOT in vain. Although the actual link juice or link power of these sites may be lower it makes your linking profile look natural and normal and that is critical and lets not forget traffic you will still get from someone clicking on your link and visiting your web site.