These have a 2.572% chance to be the fourth card in your pack, or a 10.288% chance to be the fifth card
Turning late game manadorks into threats also lets us grind very well, while Artisans body is also probably going to be quite big on its own in a long game
Lands often have no use late game in limited, so sacrificing a couple to give yourself a few extra cards never hurt, especially when the additional costs involved "tap" and not "spend mana"
Thrummingbird is an excellent early play for anyone who's in a hurry to proliferate
Sacrificing three creatures can be a drawback, but in decks filled with tokens from Bastion of Remembrance or Endrek Sahr, Master Breeder, it becomes trivial
The explosive tempo of Legacy and Vintage, unique among Constructed formats, comes from their avenues of transmuting cards into mana Black Lotus, the Moxen, Channel, Lions Eye Diamond, Dark Ritual, Lotus Petal, Grim Monolith, Cloudpost the list goes on