It is simple, if you play against another player the reward should not be just the MGP but with small chance a card too. If you win against one player too much, then the next game won't give you the chance to get a card, like when you get less MGP too from them, you have to try it tomorrow against that player or try playing against other players. (i think i don't have to explain how good this can be to improve the number of matches of normal matchmaking)
For tournaments there is no bonus card loot, it would be too easy for those who are already playing it every week.

Possible rewards:
Hinene
Celestria
Dominy

These NPCs can be found in Wolves' Den Pier. For PvP you get PvP related NPC cards.