Waste Not makes an opponent's discarded card produce something for you: a creature becomes a Zombie, a land becomes two black mana, and a noncreature nonland card becomes a draw. The two-mana enchantment gives a discard deck ways to develop its own resources while reducing opponents' options, rather than relying on hand disruption alone to finish a game.
Windfall in a blue-black deck can create a burst of different triggers across several opponents. Rankle, Master of Pranks provides a repeatable discard option through combat, while targeted discard can remove a dangerous card and collect its associated reward. You do not choose what reward a card produces independently; its card types determine which ability applies.
The mana from discarded lands arrives when the relevant triggers resolve and normally empties as steps and phases end. Have an appropriate use ready instead of assuming it can be saved for a later turn. A discard effect during an opponent's end step may provide tokens and cards that remain useful, but mana generated there needs an instant-speed outlet or another way to preserve it. Waste Not is most effective when discard happens repeatedly or in a large enough event to create a strong follow-up, with enough actual threats and card access to keep the deck functioning after opponents' hands are empty.
