You only need four creatures to die in a turn to flip him to his One-Winged Angel form which not only gives you that effect as an emblem, but can also sacrifice any number of creatures to draw you that many cards
A cousin to mechanics like Offering, Casualty, Devour, and Exploit, Bargain is an additional cost that could theoretically be attached to any nonland card type
With a now wide variety of spells, creatures, lands, and colorless options, green draw cards can keep up with most other colors
However, if youre playing a commander that doesnt generate value, such as [c]Kaalia, the Vast[/c], youll definitely want to include many ways of drawing extra cards
In other words, he's happy to cast some learn instants and sorceries to recur creatures repeatedly (that can also learn, in many cases) to go grab Lessons to cast to recur creatures repeatedly
After all, if you're going to lose a creature anyway, at least gain something from it