Linux

Most people expect a GUI interface to get into their desktop. But you dont have to use one if you dont want. SDDM can log into any desktops you have - KDE but also Gnome or XFCE etc. It can also help select X11 vs Wayland sessions.

There are alternatives like LightDM if you dont like SDDM. Or TTY is fine too. But generally they're not large pieces of software and while they are undoubtedly bloated from what they could be, they are still small and lightweight in the era of Tbs of storage and Gb of memory. The savings you'd get in not using them would be small on the scale of the rest of the OS. Obviously they're useless for none GUI machines / servers.

They're called display managers because historically the concept was added to X11 system where you'd have a stand alone X terminal running locally for the end user with an X server which would then connect to an X display manager on a central machine. This was in the Unix days and shared spaces like governments, universities or corporations and the set up was potentially less hardware intensive allowing cheaper X terminals and an expensive central server.

The concept has gone now - PCs are vastly more poweful and can easily run the entire OS locally, and thin clients are the modern set up if you do want terminals/clients and central servers. The most common scenario is now the display manager running on your local PC, alongside everything else and essentially replicates the TTY login in a GUI form. So yes its basically a session manager but the name is historical and probably won't be going anywhere fast.