Understanding the phasing allow you to implement clever tricks
Sidisi might be Commander-eligible, but it's better in the 99 of a deck
For example, if you somehow used Souls of the Lost to craft Mastercraft Raptor (perhaps because it became a copy of a Dinosaur), its power while in exile is equal to the number of permanent cards in your graveyard, and Mastercraft Raptor's power will change as the number of permanent cards in your graveyard changes
And, if you're just getting started with Magic's digital younger sibling, try out our guides to MTG Arena codes and the best MTG arena decks to get the ball rolling.
With Fallout and a bunch of surprises in-universe, I feel inspired
+1/+1 is not enough to alter the vulnerability of Gruuls creatures, since one of the decks few bright spots is a natural resilience to damage-based removal like Deafening Clarion