XEmacs was my first Emacs. It supported Windows properly (GNU Emacs relented many years later and now treats Windows quite well) and graphical possibilities that made many users recommend it to newcomers like me.
I think it also had package management at a time when the GNU Emacs side thought that was decadent.
In the end it was a bit like the gcc/egcs situation: GNU learned something from the schism and started to take users' wishes seriously, and shortly thereafter there was no reason to continue separate lines of development.
To add more detail: You can split a window into multiple subwindows, and each subwindow can show any file. If you want, you can watch different parts of the same file in different subwindows (each subwindow can be scrolled independently). This is something that emacs does better and more flexibly than many supposedly advanced editors and IDEs.
For historical reasons emacs uses its own terminology: a window is called a "frame", and what I called subwindow above is called a "window". Instead of a file, in emacs you talk about "buffers", although a buffer is not exactly the same thing as a file, since you can (and do) have buffers that are interfaces to other emacs functionality, not linked to any particular file.
Nice blast from the past -- I used xemacs from around 2000-2005, including for reading email and IRC, because that was a thing people used to do. I had a setup where I used it graphically at the machine, but could also ssh in and attach to the same process on the terminal, which I think was why I wanted xemacs rather than GNU emacs.
I think it also had package management at a time when the GNU Emacs side thought that was decadent.
In the end it was a bit like the gcc/egcs situation: GNU learned something from the schism and started to take users' wishes seriously, and shortly thereafter there was no reason to continue separate lines of development.