They are a mixer that digitally controls temp and flow balance. The controls also don’t need to be inside the shower, as they are a digital control, either wired or wireless.
The Mira one I’m contemplating controls both the shower and bath filler to your selected temperature. You can also set the bath level required.
Do I need it? Of course not. I do like the idea of accessing the controls outside the shower, setting the required temperature and it tells you when it’s ready. The bath filler removes the need for taps, which in my bathroom helps a lot. I can place the remote button in a place accessible from the bath too. There is an option to connect via an app too, so remote filling the bath, or starting the shower as you get out of bed!