From the Rubble Repeated reuse of your creatures and no additional cost: These are excellent traits for reanimation
You're either ramping towards your big threats or putting another big threat into play for free, and either way it's advancing your game
However, Krenko's best feature out of everything: he's a token machine
Once we have tokens we're going to pump them with cards like Jeskai Ascendancy, Surge to Victory, Warlord's Fury, and Balmor, Battlemage Captain, all while Narset pumps the team with prowess and casts additional spells from the graveyard
A two mana 2/1 with the Vipers ability is a pretty nice two drop for Limited, as it is reasonably likely to chip in some extra damage all game, so the fact this can also bounce something is nice Cleave Shadows is certainly somewhat situational, but I think youll be surprised by how many board states there will be where it lets you kill something