The motivation boils down to trying to make runs with the same starting seed feel "similar" in meaningful ways. It's "better" in a vibes way if both you and I were offered the same card choices in runs with the same seed, even if we took different amounts of turns on the first battle.

The main reason to allow users to set seeds manually is to allow players to share seeds among themselves. And the reason that players want to share seeds is because players will find exceptionally rare seeds that other players might want to try out. This sort of exceptional rarity might take the form "the first shop in act 1 sells a relic that gives you 2 potion slots, then the act 2 ancient offers a relic that gives you 4 potion slots, then the act 3 ancient offers a relic that fills all your potions slots at the start of every combat". However, when players share seeds, they aren't sharing the exact series of inputs they performed in that game. This means that the relics that have been "randomly" offered like the ones above need to be offered on the same seed regardless of any prior player decision. And player decisions can generally cause the RNG to advance an arbitrary number of times. So this means that you want to have entirely separate RNGs for every thing that the player has any power to influence, because this makes the randomness of a single seed more usefully reproducible in practice.