Commenting on blog posts is first about adding value to a conversation, but on the same token, is also a great opportunity to gain a good valued link back to your own website.
If you have good content to share that truly adds value to the conversation, a blog posting strategy can form part of your backlink building strategy. Choose the right forums and blog posts in your industry and niche. A link to a beauty salon from a heavy machinery parts supplier is not only weird, but just reeks of a spammy link and adds no value to the reader or the website owner. It will either be not approved by the webmaster or worse, if it does, you risk Google applying a penalty (Read Penguin SEO Update) to your website.
Use Drop My Link to search for blogs in your niche that you can comment on – handy tool!