Not only are you showing goodwill when you add value to someone else's post, but you may also:
- bring the blog author to your blog or site
- bring other reader's of the blog you commented to your blog or site
- create relationships that could lead to guest posting opportunities
It does not require a lot of time to do this. Schedule 20-30 minutes every day visiting blogs within in your niche or related to your niche, and you will see that "what comes around, goes around."