What is link building? Simply put, it’s getting other websites to link back to yours. But not every link matters and not every link will yield a positive result.
Google spiders love links, and the more backlinks they find, the more likely they are to find you. It is a lot like finding a place, the more roads and paths that lead to it, the more likely it is someone will see it. If this place is in the middle of nowhere with no pathway leading to it, chances are no one sees it anytime soon.
Backlinks also provide you with reputation. At least in the eyes of Google, your reputation is informed by how many others point to you. Think of it as a popularity contest, where the more popular you are with others (the more websites link to your site) the more important you must be, and the more important you are, the more you can be trusted, and so the higher your rank. After all who are you more likely to believe, the source no one links to or the one with hundreds of links?