Because when one side gets dirty, you can take it apart and turn it over.
Then, when that gets dirty, you can swap end over end.
Then, when that gets dirty, you can turn it over again.
That gives you four clean corners
Just like your underwear, there’s normal, back to front, inside out, inside out and back to front…
(Only jig users will understand this - if you just plonk things in your machine any old place - well, there’s no hope for you)