It's quite common for a site to be referenced by more than one domain name. In fact, most sites are referenced by at least two: a www version and a non-www version. These are usually set up to reference the index page on a site and produce the same results for a searcher.

It could, however, be argued that these are these same domain names. So the question remains: why would someone want to have more than one unique domain name for a single site?

Search engines - First, let's take a brief look at search engines. In the past, it was a very common spamming technique to purchase dozens, hundreds or in some really gross cases, thousands of domain names, all referencing exactly the same site. These were all submitted to the search engines, and many of them were indexed and blindly added to the results. This is how many questionable sites used to get top search results very quickly and inexpensively.

The search engines have apparently caught onto this technique. At the very least, it has become common knowledge that this kind of spamming is not tolerated (sometimes common knowledge can be just as effective a deterrent as actual enforcement). I know that in the past it was normal to find many sites of different domain names but identical content in search engine results; today it's far more rare.

In fact, the top search engine, Google, bases it's ranking scheme on quality of links. What this translates to is you must get popular (higher ranking) sites to link to your site to raise your ranking. Thus, it's a better strategy to get as many links to a SINGLE domain name than to many different domain names.