Learn was showcased on the Pop Quiz card, which lets players draw a card before also Learning - which is either discarding a card to draw a card, or fetching a Lesson from "outside the game" (in play, this means the player's sideboard)
Notably, this card has no limitations, so if youve spent your turn producing ample Elf tokens or dropping tons of one-drop Elves (both examples with summoning sickness), you can tap them all down for loads of mana
Ive watched videos and streams where players try to draw cards against her, Ive attempted it myself, and Ive had a Narset out when an opponent tried to draw cards at least 100 times in total
Wrenn and Six can use its minus ability multiple times before leaving the battlefield something very few Planeswalkers can boast
Even if Essence Channeler gets removed, you can just move the counters over to another creature so all the stat increases don't go to waste, turning a completely different creature into another threat your opponent will have to deal with