You can get quite far without buying a customized Webconverger.
I just created a boot tutorial to give new Webc users some starters.
So why buy? You get your very own build (architecture), your own packages and fixed settings (more secure). And of course, you support Webconverger as a free software project from Debian.
Customers also buy a customised version of Webconverger for Webconverger's support. It's easy to charge for a build of Webconverger, though it's difficult to charge for support. A lot of support happens even before a purchase. I'll tell more another day.