It’s very nice to not have a dozen different versions of the same distro to parse through and figure out that are simply the same distro with a different DE. Moreover, very few of them offer this many options.
Cachy could be doing a better job explaining what the user is looking at here and who each of these is for. Pretty easy to sum up in 1-2 sentences…
Simplicity for users and support staff.
I don’t think that the average user cares for customisation far beyond wallpaper, and perhaps theme. Note I’m not saying average Linux user, I mean average person using a device. Think your aunt who can’t plug in the printer. Faced with too many options people shut down.
If you have a distro and need to offer support for it, it also helps if you can write guides and instructions for a single type of scenario. With Windows you can say “right click the start menu, click device manager…” etc, but that’s not quite as easy on Linux. You can always direct people to the terminal, but again, the average user is likely to balk at the idea.
Choosing a dedicated DE means you have less to maintain, and less to support, and can focus your efforts elsewhere.