Getting lots of inbound links will certainly benefit your search engine rankings, but there is another great reason for trying to build up your inbound links.
The more links you have around the Internet the more traffic you will get from non-search engine sources. It's not always a good policy to rely on a single source, e.g. free search engine traffic, for all of your visitors. If the search engines change their policies your top ranking position may vanish overnight along with all the traffic that it brought to you.
If you have dozens, hundreds, or even thousands of links coming in from other sites around the Internet the chances are that these links will, over time, bring you a lot of your visitors. Even to the point where you get far more traffic from your inbound links than from any other source, including the major search engines.
For example, if you were running a car hire business you could probably get lots of free traffic by having a prominent link on the sites of relevant hotels, tourist attractions, airports, vacation sites, etc. Obviously, these need to be relevant. If your car hire business is in Tibet there's little point in having a link on a Web site for a hotel in Miami.