It's required by the software. It is available to editors who may use it (although the vast majority of the time they will NOT respond via e-mail to site suggestions.)
It doesn't have to be on the suggested website. Remember, the ODP concept consists of volunteer surfers working together to find, review, categorize and list sites. The expected use of site suggestions is for surfers to tell us about good sites they found.
We aren't going to OBJECT if a webmaster tells us about his own unique content. But be careful NOT to think of that as normative, let alone as the only use of the "suggest a site" functionality.
And, of course, the "normative" behavior would be for the surfer NOT to HAVE an e-mail address on the website.
There is, however, no rule AGAINST having a website on the suggested domain, either. In general, the ODP process simply isn't driven by that kind of irrelevant arbitrary rule-making.