Enters-the-battlefield abilities are great in all formats, but especially in multiplayer formats like Commander. The more players there are around the table, the less likely it is you'll get to untap with that big creature you played last turn. That means you want your creatures to affect the board on the same turn you play them—which they will, if they have an ETB ability.
Once you're playing a bunch of ETB creatures, how do you get even more value from them? You could let them die, and reanimate them for extra triggers. You could flicker them. You could run Commander staple Panharmonicon in your 99.
Or you could play Panharmonicon in your command zone.
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