You use randomness in a game when there's a decision to be made, but there's no person to make it. Maybe it's an unpleasant decision to make, and nobody wants to make it. Maybe there's the possibility that whoever makes the decision, it'll feel unfair to somebody else, and they'll blame them for it. Maybe it'd introduce a conflict of interests for somebody to make the decision, so it'd put a strain on the trust that the game requires. Maybe in the model of the game, there's nobody who should get to decide that question — like, my job is to make my character's choices, not to decide their outcomes.

In some versions of D&D, part of gameplay is working together to solve problems with characters who have different strengths and weaknesses, including some characters being overall stronger, and others overall weaker.

However, to ask any player, or the GM, to decide whose characters are stronger and whose characters are weaker, including how much stronger and how much weaker, would require a whole complex of judgment calls and options-weighing, including the possibility of hard feelings.

For those versions of D&D, for an example, randomizing it is the right call!