UX, speed, simplicity, lightness. Applications ran without talking to the internet, asking if it's ok to run it, telling me it's probably unsafe to run it, telling me I need to update for security reason, telling me I should play candy crush, letting me search my files without adding recommended noise somewhat supposed to be relevant to what I did 3 days ago. I could go on. I just want to stare at a blue flat colour knowing tomorrow it will be just the same. /s
Depending on the "Distro" and compile options/build flags/included libraries MATE is as fast and light, since at least 6 years, actually. I think the first distribution which showed that to a general audience was LMDE a.k.a. Linux Mint - Debian Edition.