Thematically, youre building up the story of another character, taking a lowly Llanowar Elf up to a massive 10/10 creature, and now you can tell their exploits throughout time (or at least until you win the battle)
Ancestral Recall Any list thats considering all the mtg best blue card draw in the game that doesnt have this as the best one is just plain wrong
Two turns later, they stop, spells keep flying, and Sentinel starts refilling your hand
Zen Takahashi Having only been on the opposing end of an Esper Sentinel, I have found the card to be an absolute pain to play against, but I do really like the design of the card
(For those of you who arent familiar with cumulative upkeep, this effect triggers at the beginning of each of your upkeeps