- stupid question: what exactly does this do and why do I need it?

Logitech mice and keyboards have extra features that the OS would consider non-standard; you can customize a lot of behaviors as well as map extra buttons to specific actions. For some reason, Logitech decided not to store customization settings on-device so there is a background daemon that must run on the host. Also inexplicably, it is an electron app with a 600MB install footprint requiring online connectivity (apparently there's also an offline version, weighing in at 1.2GB). Also not available on Linux.

You usually don’t _need_ it but there are a couple things you can’t do without it. Pairing a mouse/kb to their usb receiver for example is not possible without it.