I don't think they'd be so popular if they weren't useful.
Why should the user be required to reinstall their whole OS? I don't think they are: it seems relatively straightforward to change DEs on Ubuntu at least.
On the other hand, if someone knows they want Ubuntu with KDE, why should they have to go through a regular Ubuntu install just to do the post configuration themselves? Plus, maintainers of these offshoot distros can potentially more deeply remove dependency on the default DE.
I think focusing on differences in system software is less illustrative than looking at the out-of-the-box user experience and capabilities. A changed DE is a pretty huge practical difference.
This line of thought does really underscore how nebulous the definition of an operating system really is. Pour one out for GNU being totally subsumed culturally by a Kernel that everyone sees as an OS.