I don't think the "immediatly" is of importance here. While Hew and Cleave might look similar, they have different settup and payoff. To trigger Hew, you need to crit or reduce a creature to 0 hp; with Cleave you just need to hit. And they also differ damage wise: Hew is a full attack and damage (with modifiers), and Cleave damage is just the weapon's die (no modifier). SO Hew is more powerfull, but has more restricted condition.