Why random numbers?

Yes, that's a good question... Do you really want an answer? OK, I'll try to give you some keys.

If we don't use random numbers, the game will be entirely determinist (i.e. a given situation will always evolve in the same way) *without* players.

we consider absolute time as a random number (because we don't control it). So we suppose that situations are not a function of absolute time, or only of duration.

But it won't be a game if there aren't players, will it? With the first player, we lose the determinist behavior of the game (we suppose that the player is active, so he/she interacts with the game...). The player acts when he/she wants and does what he/she wants. With time, the player can understand how the game works (and even more with the source :) and uses these informations to cheat. With several players, each player acts when he/she wants and does what he/she wants, but there are as many degrees of liberty as players, so it would be more difficult to have a full understanding and knowledge of the game (i.e. to find the formula which manages the game and describes its future). But, if players decide to cheat together, they'll control the future. That's the first reason of having random numbers: it increases the cheat difficulty.

In the real life, a classical philosophical question is "Is the world completely determinist but chaotic, or do we have some freedom to do what we want when we want?". The answer does not matter for us. The only important point is: the world is unpredictable for us. So if we want to do the same in the game, we can either code extremely complex functions (impossible to reverse to cheat and able to create all or many various situations), either we simply use random numbers and keep human code (i.e. simple but buggy :p). That's the second reason.

The use of random numbers create such a chaos that even we will be surprised by the game evolution. It's even more true for players. That's nice for game-play: various and unpredictable situations. That's the third reason.