http://bit.ly/1i0DEbM The most important reason you want to start an link building campaign is to improve your search engine rankings and get free traffic from Google, Bing, Yahoo and all the other little search engines of the web when people search for your topic. Links to you from other sources says to their algorithms, "Hey, look at me, I'm important when it comes to this topic."
The second advantage of a link building campaign is simply exposure of your name and becoming known as an expert on your subject matter. If you're the best tomato gardener in the world and garden bloggers start linking to you because they like your information then their readers are going to see that and come check it out because they trust their opinion, and in turn those readers might mention you too, and so the cycle goes.
Link building helps establish your value. People don't like to link to things that are spammy, so when you're getting authentic links from real sources it shows readers and search engines that your content is valuable in some way. After all, no one wants to read heaps of keyword filled spammy junk online, they want solutions to problems, tidbits of interest, and maybe a little bit of entertainment when they're coming to your website.
In many cases building links to your website can be completely free advertising and marketing for your website. Why pay for high priced banner ads and crazy services when you can build an online reputation that will last for a long time to come? All those little links you gain today will continue to serve you in the coming days, weeks, months, and years of business online