When search engines calculate the relevance of a site to a keyword, they consider the number of QUALITY inbound links to that site. So we should not be satisfied with merely getting inbound links, it is the quality of the inbound link that matters.
A search engine considers the content of the sites to determine the QUALITY of a link. When inbound links to your site come from other sites, and those sites have content related to your site, these inbound links are considered more relevant to your site. If inbound links on sites with unrelated content, they are considered less relevant. The higher the relevance of inbound link, the greater their quality.
Search engines want websites to have a level playing field and look for the natural links built slowly over time. While it it fairly easy to manipulate links on a web page to try to achieve a higher ranking, it is a lot harder to influence a search engine with external backlinks from other websites. This is also a reason why backlinks factor is so highly into a search engine's algorithm.
Another reason to achieve quality backlinks is to entice visitors to come to your website. You can't build a website, and then expect that people will find your website without pointing the way. You will probably have to get the word out there about your site. One way webmasters got the word out used to be through reciprocal linking.
Reciprocal links are one of the targets of the search engine's latest filter. Many webmasters had agreed upon reciprocal link exchanges, in order to boost their site's rankings with the sheer number of inbound links. In a link exchange, one webmaster places a link on his website that points to another webmasters website, and vice-versa. Many of these links were simply not relevant, and were just discounted. So while the irrelevant inbound link was ignored, the outbound links still got counted, diluting the relevancy score of many websites. This caused a great many websites to drop off the Google map.
We must be careful with our reciprocal links. There is a Google patent in the works that will deal with not only the popularity of the sites being linked to, but also how trustworthy a site is that you link to from your own website. This will mean that you could get into trouble with the search engine just for linking to a bad apple.
Building quality backlinks is extremely important to Search Engine Optimization, and because of their importance, it should be very high on your priority list in your SEO efforts. We hope you have a better understanding of why you need good quality inbound links to your site.