Some distros have editions with a WM (usually i3) as a default, yes. These editions tend to come with some basic config so it's more usable out of the box. But you can also install WMs side by side with DEs and then switch in the login manager (GDM, SDDM), just the same as you can install multiple DEs on a system. You could also install a headless version of a distro first and then install only the WM and whatever other tools you want on top of that. Basically all system settings can be changed through config files or CLI programs, for some things like audio and bluetooth there are good DE-independent settings programs like pavucontrol.

You can also replace the WM built into KDE (kwin) with i3, for example, but that's pretty messy, IMO.

As for advantages, WMs are usually very keyboard driven, you pretty much never have to touch the mouse. They also tend to be fairly light weight and use little RAM. My favourite i3 feature is that workspaces are per-monitor, so I could easily move multiple windows between monitors and not lose the way they are set up.

As for disadvantages, changing any system settings tends to be a research project, because there is no centralized solution, it's even worse than Windows in this regard. Personally this is the main reason I switched back to KDE from i3. I could also never get theming to work quite right.