The passive alone is useful, and it also allows you to get a second use out of a creature, whether because it has a good enters effect, or because it hits hard
The Strong Ultimate is Immediate and Doesn't Kill Sorin - There are a few modern planeswalkers, such as Gideon, Ally of Zendikar, that could be dropped quickly, and used with it's ultimate as soon as it arrived, but you had to kill it
It has a high buy-in but Gaeas Cradle shows us how high it could theoretically get (not that a card that is multiples in Legacy is comparable in demand to an EDH card, though there are fewer Sanctums than Cradles)
Kogla and Yidaro have trample and haste on EtB, while Grunn, the Lonely King is amazing when attacking alone as it doubles its stats and can become huge if we use its kicker cost or combined with our +1/+1 counter strategy
Put a +1/+1 counter on target creature
A manifested creature card can be turned face up any time for its mana cost