Keeping a creature around and getting a little bit of card advantage is a huge benefit in the long term and since it can be done at instant speed, you can use it anytime you want (so long as Soul Diviner doesnt have summoning sickness)
The "Power 9" (like the Black Lotus) offered such unfair advantages they were banned from competition
Those days look to be behind us with Day of Judgement returning to Standard, providing a hugely important piece to control decks, allowing a reset of the game that can often be a three, four, or five-for-one (or more!)
Without flash Hollowhenge Overlord would probably die right away, but sneaking it in on an opponent's end step makes it that much more likely you'll get at least one trigger from it
( Murders at Karlov Manor will have basic lands.) It will be traditional foil 20% of the time and non-foil 80% of the time
(6/18/2021) A spell's mana value is determined only by its mana cost