* It helps to get your site listed in the search engine.
* It helps to boost your position in the search engine.
* It helps to build small streams of traffic to your site.
Links to your site are normally given by also giving a link from your site to the other one. These are called reciprocal links or link swaps. And naturally there are a few services available to automate the link somehow.
Some of these services will automatically add the link to your site and the other site once your link request is approved (through some software to be installed on your site).
Some will simply point you to sites which do use link swaps and who are interested in hearing from you.
Some will also check that the link to your site remains in place, and email you if it disappears. It's then up to you to either contact the owner of that site to find out why the link has vanished, or to remove the reciprocal link on your site.
But there is one thing they do not do, and which you need to watch for:
How would a visitor to the other site FIND the link back to your site?
Because you can be sure that if a human visitor cannot find it, then it's unlikely that a search engine will.
Let me give you an example: Andrew was using the service at LinkMetro.com to get links to one of his sites. Someone had a site on a related topic, and they requested a link back to Andrew's. He checked the link back to his site, and everything looked OK. The other site had requested a link back to their homepage (rather than another specific page), so Andrew checked out that home page.
What did he find?
* No links to the "link directory".
* No link to a "related sites" page.
* No link to a "resources" page.
It seemed that the link directory on that other site was not linked from the home page of that site.
The other site was requesting inbound links back to its home page, but effectively hiding the return link from the search engines and from website visitors. And that makes the link back to Andrew's site useless - it's like that link doesn't even exist.
So next time you get asked for a reciprocal link, check the route that people and search engines would use to get from that site over to yours. You might be surprised what you find.