Link building is perhaps the most important element when it comes to successfully marketing an online venture.  One can have the nicest website, the coolest domain name, an offline marketing campaign and a killer product, but without off-site links, your website is next to invisible.  Although there are a significant number of factors involved in how search engine rankings are decided, the basic idea is that the more links a website has to it (also known as backlinks), the higher up the rankings the site will appear for a phrase.  This is a gross simplification, but having more inbound links is always beneficial.