By some method, search engines can look at a page on a website and immediately tell how many sites are linking in to it. This shows the engine that other sites find it valuable. Not only that but the search engine actually reads the text that makes up the link from the outside site, thus determining that the site actually is telling its customers what they will find of value if they click on the link. So not only do you have 100 links pointing to you, but also descriptive links. This means that you get more traffic.
More traffic means more potential sales.
You can put together a great site with great content that other want to link to. You can trade them, but this is time intensive. Or you can submit them to FFA pages. The problem is that most FFA pages end up dropping you as they add others. So submitting has to be regular and can be time intensive.