The key difference between all the Ubuntu versions is the desktop environments which they promote, for example Ubuntu is Gnome 3, Xubuntu is Xfce, Lubuntu is LXDE and Kubuntu is the Kde Plasma, on top of that they each bundle a set of packages which they think their communities will like and possibly make tweaks to their distributions which they think need to be made.
In my personal opinion, which of course everyone is entitled to, I think fragmentation of linux distributions isn't a good idea, each distribution I've tried before Lubuntu I've had to uninstall what I would call bloatware, but basically that's applications which I'd never likely use, with Lubuntu I've not had to uninstall anything, I like the idea of just enough of an OS to get me online,
I hope this helps, if I've misunderstood your question in any way please let me know and I'll gladly try to help in any way I can, many thanks

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