I imagine the reason for the difficulty hike and requirement to rekill the bosses each time is because of the gem rewards for getting past each boss without losing any health... which is also sort of a thing from Dead Cells... I will say though, I don't think the rewards are worth it at current for such a difficult task, especially since unlike in Dead Cells, you have to beat the preceding levels and the boss without getting hit, as opposed to just the boss. Also since people seem to gravitate to a single weapon, most of the upgrades feel pretty pointless, since there's not much reason to buy anything, but your desired weapon, health, the recovery potion upgrades and maybe... The dive and torture items...