Kinda as a 'just-in-case' case. If you just so HAPPEN to find this Pokemon in the wild (through some in-game glitch or by Gameshark, etc.), you won't be stuck with an uncatchable PKMN or a 100%-catchable one. If there's no data for that, then there'd be "NULL" data added into the formula, which would basically produce a random and potentially dangerous situation.
It all has to do with the coding of a game. Computers have to follow basic instructions, and formulas work better when there is data to input into them - even if the calculation is never used - than IN CASE SOMETHING HAPPENS where this formula gets used unintentionally.