Gnome-flashback by default is an old-school Gnome DE (Desktop Environment) that comes with a simple, conventional WM but allows you to swap in any WM you like while it operates in the background. Mainstream Gnome Shell DE is inextricably tied to its WM so you can’t swap into that. So with Gnome-flashback you can swap in i3 and get a curated Gnome DE with your own (i3) WM.

It means you don’t have to reinvent everything that makes a DE just to use i3 WM. You get things like the Gnome settings GUI including monitor configuration and restoration on hotplug; clipboard manager; theming; audio/brightness hotkeys just work; USB drives automount, and more. Lots of convenience and utility you want and need but otherwise have to identify, install, configure and set up manually. Without using an already curated DE you have to reinvent one, or at least reinvent the parts of one you can’t live without.

Gnome-flashback is not the only DE that allows swapping in a different WM. My own experience has found it a bit of a PITA every time I try to use it on an OS with an updated Gnome version, requiring poking around, searching and debugging. Sadly, there seem to be limited options for low effort, well polished, curated i3/sway DE.