1) to switch off the whole circuit to "zero current" consumption when H=low, by preloading node Z to high. H=≈ "latch enable" (pos. logic)
2) positive feedback to increase gain
3) usually called "hysteresis transistors". Here they are used to preload the output nodes to "high" when H="low", to change the hysteresis during its transition from "low" to "high" and provide the latching at H="high".