It’s prettier than a TTY and you can pick whether you want a Wayland or an X11 session without having to know the correct startup commands. You can pick between different desktops too. And a Display Manager can offer on-screen keyboard and touchscreen support while a TTY can’t (at least GDM does, I’m not sure about SDDM off the top of my head).

Aside from that whatever command you are using in the TTY to launch Plasma might or might not be the same commands SDDM uses, which might or might not lead to issues in setting up the environment. If your environment is fine and you don’t care about having to use a physical keyboard then of course you can remove it. It’s not exactly load bearing.

kde is making an SSDM replacement right now, not sure when it’s going to be ready for general use much less early adopters

It’s interesting because instead of “display manager” it should be “graphical login manager”, and the current “login manager” should be called “session manager”. I don’t know the origin of the name, though.

On-screen keyboard was already mentioned, but there are some other small things that might be useful for some:

Reboot/shutdown without having to login (Your husband/wife/partner can shutdown your computer without first having to login and be greeted by the porn folder on your desktop…nah seriously, this can be useful at times when your turn on the computer, get called away and someone else can easily shut down the computer after you didn’t return for some hours)

Keyboard language selection before password entry. Very useful in multi-language households/companies.

The WM selection also allows kiosk-like behaviour in special cases…like you don’t start a WM but start in kodi media player for a movie evening or you create your own WM session file for a single game that runs as soon as you login.

I prefer the tty over session managers because I generally throw in a system update before starting the gui in case it warrants a reboot. Also if you’re feeling spicy you can tell it to load the desktop on a specific GPU with $ DRI_PRIME=1 startplasma-wayland, for example.

I like the extra control basically.

I use SDDM to log in to Sway. Mainly because I started on KDE and am too lazy to figure out a different login manager

i mean, most of them are celled “display managers” – lightDM, gDM, lxDM