Short answer: (Let me explain this with a little deductive reasoning.)

The more sites that link to this site, the higher this site will appear in search results. The higher this site appears in search results, the greater the likelihood that the site will receive visitors. The more visitors to this site, the greater the chance that a visitor will choose you as a service provider.

Medium length answer: It is not likely that every service provider (on this site) will be able to get as many inbound links to their site as I can get to my site (assuming I can get all my service providers to provide an inbound link). If we make this site a "clearing house" of sorts and make sure that it has lots of inbound links then the argument for the short answer will follow. So, instead of working individually on your own link exchanges, become part of a larger group that is consolidating its efforts.

Longer answer: While you may think that a link on your site to this site is unnecessary due to the fact that the visitor is already at your site, this is overlooking the main point. The point is not that the visitor on your site will take the link to this site. The point is to try to capture those people who have not found your site by having them find you through this site as described above. A secondary point to consider is that you may not be the right provider for a particular visitor to your site; the link to this site can provide that visitor with the connection to a provider who is the right fit. Finally, we are all in this together; if you are truly committed to making the world a better place, then you need to be willing to take steps to make that happen. By linking to this site you are helping me to fulfill my purpose in the world (and therefore helping me to make the world a better place).

I hope this information was helpful. Please forgive me if it was too heavy-handed and/or more than you really needed to know.