Red Hat, Intel, AMD, SUSE, Samsung, Canonical etc.

Obviously, a lot of paid work is done on low-level plumbing, because Linux is hugely profitable as a server, infrastructure, or embedded operating system for a lot of companies. Graphics infrastructure (KMS, Wayland, X11, etc.) is useful for companies who embed Linux in various appliances (set-op boxes, etc.). Red Hat probably invests in GNOME because for Red Hat Enterprise Linux Workstation and dogfooding.

>For what reason?

If your programmers can make your project their job, you don't have to worry about the amount of time they have to contribute.

RedHat would be one company that pays for that. FOSS has yearly report on such things, if you want to google it. (I am too lazy now)