Does "Firefox" sound corporate? It all boils down to brand familiarity. "Seamonkey" was the old code name for the Mozilla Suite, and it becomes the new name for the product in the same way that "Mozilla" (the old code name for Netscape") became the brand name.
Seamonkey is not there to compete with Firefox, however. It is a community project built by volunteers who were disappointed by the Mozilla Corporation's decision to scrap the integrated suite. Luckily, as the code is open source, Mozilla simply handed out editing rights, hosted the project on mozilla.org and let the team coalesce around the new project.
Seamonkey is what Mozilla 1.8 or 1.9 would have been if Mozilla hadn't stopped development. It places the core rendering engine used in Firefox 1.5 into the Mozilla suite wrapper.
Seamonkey serves many purposes: it provides a continued upgrade path to individuals (and especially corporations) who use the Mozilla Suite or Netscape 7.x with the integrated mail component or other parts not offered by the Firefox/Thunderbird combination.
Seamonkey also adds to the diversity of choice in the browser space, which is always a good thing. Not everyone appreciated the abandon of the Mozilla suite and many appreciated the all-in-one nature of the earlier product. I would hazard a guess that the non-Windows versions of Seamonkey will be most important. It will remain a niche product as it has nowhere near the same visiibility as Firefox, but it's great to see that the old code is still being used.
Some versions of the Mozilla suite had an easter egg which gave a fully-animated ASCII art representation of a kitchen sink with running water in reference to its critics that the product was too bloated - I wonder if the Seamonkey team has retained it?! :)