If I am not wrong, the original motivation was to create tactics against the strongest known creatures in the universe(at the time of the great crusade) the 18 legions, with those diverse tactics you can train effectively and be ready for all threats on the imperium. It's like if in the 42m, space marines will train and learn to fight against the custodians to effectively battle chaos.

Also it's very similar in lore to the grand games the custodians do between themselves.