They offer two things: a deck-building challenge, and consistency
Whereas previous games would play up the narrative that your team stood against the world, pitting you against gauntlets of opponents and challenges without rest, every few minutes in Alola turns up a new friendly face
Some people just wanted more cards, not realizing that this was not to their advantage, but others realized that most decks were not designed to be able to take full advantage of a second draw every turn
Dark Sage, by contrast, has very convoluted conditions to summon: the player has to Tribute a Dark Magician straight after using Time Wizards effect and getting the associated coin toss in their favor
Soul of the Harvest may not enter the fray early on, but when it gets there it makes Selvala better, it draws cards when we flicker it, and it has trample built in to allow us to swing through blockers
It would be better to find another helper