What happens if you enable it?
I don't think both are necessary. This is why I don't like UEFI. Things become difficult.
Since you just want Ubuntu MATE and not Ubuntu MATE plus Windows it doesn't really matter if you install in UEFI mode or Legacy mode.
I install in Legacy mode because I always install multiboot setups. I find it to much work to set up multiboot with UEFI.
My advice is: It doesn't matter if you install in UEFI or Legacy mode. Just let the installer do its thing. I am pretty sure the Ubuntu installer is able to set up a fully working Ubuntu MATE installation as a single OS.
UEFI gets difficult in multiboot environments, but shouldn't pose a problem with a single OS.