To exchange links between sites is one strategy that's being done by many webmasters to increase a site's link popularity. This is popularly known as reciprocal links exchange.
What one commonly does is simply send an email to another webmaster, asking his permission to exchange links between their two sites. If they both agree, they then put links towards each other's site. For just one link request, this can be easily done.
However, manually exchanging links with 50 or more sites, is a very tedious process. Exchanging links manually is indeed the most dreaded work that many webmasters despise. And this is really true.