They want two players with the same seed to have a similar gameplay experience in terms of challenges encountered.
For this reason they want to isolate the RNG that procedurally generates maps, card rewards, shop contents, from other RNG streams such as the RNG used by random enemy attacks in battles, random event outcomes, etc.
One example of the wonky things that are possible when there is a single RNG stream is speedrunners manipulate the RNG state. For example, in Super Mario 64 they can jump in place to produce dust clouds that advance the RNG state until it is in a favorable position.