both ?
One side for smooth bases and one for matt.
I'd use the smooth side for pla and flexibles and maybe the ro0ugher side for more problematic filaments like nylon and abs.
The theory (I just made up) being that when a thin layer of material shrinks - ala nylon and abs - it's a horizontal pull. So the 'rougher' side with lots of microdimples should give better adhesion for the filaments that shrink.
Essentially it's got 'tread' to give better grip.

Assuming it's just held in place with easy to remove clips and you can easily flip it over - that would make sense.

For flexibles I'd just use the smooth side and no active heating.