When you add an advertise on blogs that is relevant to your own website or blog you are probably assuming that most of your traffic will come from the ad itself. This makes sense because visitors to the blog will see your ad and then hopefully take action by contacting you about your product or service or by clicking on a link which would take them to your own website or blog. This makes sense because this is similar to how advertising works offline. You place an ad in the newspaper, on tv, etc. People see your ad and then hopefully take action. However, there is one extra benefit to advertising online - backlinks!
What are backlinks? Backlinks are the links that point to your own website or blog and can be achieved in many different ways. For example, if someone likes your website and posts a link to it on a forum it's a backlink. If someone mentions you on their own website or blog then that is also a backlink. Backlinks are a good thing because they help Google decide just how important they think your site is, and the more relevant backlinks the better.
Unfortunately you can't always depend on someone else backlinking to your site, especially if it's a new site. This is where advertising on blogs has some really good advantages. For example, if you have a website it is important to get backlinks to it to help it rank well with Google - and the higher you rank with Google the more traffic you can get on your website or blog.