Single board computer projects can often be network attached devices, where operating headless is the obvious way to go. However, there are times when it is easier to things via a GUI rather than command line using a remote desktop, hence why use Mate in a headless setup. Mate is well suited to this as it because of its light resource use.
But hey, if you are happier using your Pi attached to a keyboard and monitor go with it, that the great thing about single board computers they are so flexible in the ways that you can use them.