Not sure you got the point? It's not about random generation, it's about random mechanics. A weapon does 1d6 damage, you have a 80% chance to hit an enemy, loot is randomly generated from a table, an AI has a 1 in 5 chance of executing a certain action, etc.
This applies to all sorts of games, not just roguelikes, but for roguelikes it's quite important for tactical decision-making and the feeling of fairness.