Launching a new site can be tricky. Unless you have a platform on which to announce your new site (usually an existing popular site), you'll need to go on a campaign of link building. While there are a lot of techniques for this, initially the main technique used by bloggers is to simply comment on other people's blogs, especially those in a similar niche.
This serves to get your name "out there", first and foremost. A lot of people will follow links in blog comments, so regardless of whether "nofollow" is in use on a blog, any comment has some inherent value. There is also a fair chance that a blogger will follow the links of the people that comment on their blog - and if they are in a similar niche, they may link to your blog.
That is where things start to get interesting. If you have link-worthy content, you still need people to see it in order for them to be aware it is there, and good enough to link to (this can be another problem with launching a blog - some of your best content may be posted at the start, when you have no readers and therefore not get nearly the attention it deserves).
Comments are the quickest way to get that exposure. The more you comment on other blogs, the more likely people are to find yours and link to your content.
A comment is also essentially a tiny taster of your own site. Many people's first impression of you and your site is your comments - making a good impression with your comments is incredibly important.
Comments also add value to the commentee's blog. Visitors like to read comments, and they like to see activity on a blog. It adds a sense of community to what otherwise can seem like little more than a soapbox. And good content deserves comments - a lack of comments, conversely, can indicate poor quality content.
This makes bloggers who visit the blogs of their commenters, and comment in turn on those blogs, extremely valuable. These are the people who create a good impression for their visitors. They are complimenting their commenters - saying "your comment made me want to find out more about you".
And in turn, while these people are out commenting on their commenter's blogs, they are finding content to link to (it seems to me that the most prolific return-commenters often link to some of the freshest and most difficult to find content out there).
All of this in turn brings me on to how I apply this to marketing a website or blog: I keep a list of the people in each niche I work in who "return" comments. If I'm marketing a new website, I use that list as the basis for any commenting work (on that subject: all commenting work I do is strictly white-hat - genuine, considered comments all the way, no keywords in the "name" field and no spam - quality not quantity).
My list serves as a sort of white-list. If I'm going to spend time commenting on blogs, and by doing so increase the exposure of a site, I want two things:
Essentially, this means that site launches are now relatively simple affairs. I can spend more time writing better comments on the sites that I know I'll get the best return from. I can safely ignore nofollow (because commenting can be about exposure, not SEO). I can avoid commenting on the sites of people who do fire-and-forget blogging (where they post and then ignore the post, never responding to comments or visiting the sites of their commenters).
Best of all, it is really enjoyable work - rather than trying to generate a massive number of links for SEO purposes with generic thoughtless garbage comments, I can justify spending quality time on a site, getting to know the author from their work, and engaging with them in a very real way - better for them, and better for me.