The primary purpose of link hoarding is to keep popularity up but not give any traffic to other sites, whether they are direct competitors or not. In general, link hoarding is considered disreputable because with Google's PageRank algorithm, it can actually hurt a website to not link more than it would hurt to include links.
Outbound link are used to show search engines where your website fits in a particular internet category or community; in fact, it typically boosts rankings rather than hurting them. They also let other sites know that you consider them authorities on a particular subject, as well. The more willing you are to link to another site, the more willing those other sites may be to link to yours.
It is also important, however, to make sure that your site doesn't have too many links. This can give the impression of a site being spammy or directory-like, and could also be penalized by Google. Using links within the context of your content, though, is a good way to ensure that your links serve a definite purpose and are not spammy.