In riichi mahjong, a concealed pinfu hand is worth 20-fu if it goes out on a tsumo. However, a hand with no points that has been opened gets a 2-fu bonus for no reason other than being an open pinfu. Even if the hand has another yaku to go out on (since yaku is only given for pinfu if it remains concealed), the final score is calculated with a minimum of 30-fu (due to rounding) when going out on a tsumo.
One effect of this is that it is impossible to make a 20-fu 1-han hand, since any hand that goes out with 20-fu would be guaranteed two yaku: one from menzen tsumo (1-han) and one from pinfu (1-han).
Other than that, I can't see any reason why open pinfu should be getting those two extra fu. Is there any actual reason for it, or is it "just one of those things" that's lost to the history of this game?