Why hysteresis is necessary?
Without hysteresis, computers would have no memory (either short
term or long term), and even if I were fast enough to type this
message completely before my computer forgot about it, there would
be no way for you to read it without a storage disk, which
basically works of some sort of hysteresis (most probably
ferromagnetic)
hysteresis is "a dependence on the past": it's memory.