In comments below, folks are riffing on Fr. Richard Rohr's timely claim that, "Bethlehem was more important than Calvary." Was Calvary necessary, and if so in what sense? Was Bethlehem necessary, and if so in what sense?
Absolute necessity is a tough concept, and even tougher when a creature tries to apply it to the Creator.
Even our basis for declaring the necessity of His existence seems to be contingent upon the existence of His contingent creation. If only God existed, would it follow that He necessarily exists?
I suppose logical necessity as applied to God is chiefly a matter of making sure we aren't talking nonsense. It's not God Who is bound by human logic, it's human reasoning that is bound; if I assert something about God that is contrary to logic -- something that is both true and not true at the same time in the same way, for example -- I've said something nonsensical. To say that God can't break the law of non-contradiction isn't to proscribe God's power, it's to recognize that the contrary claim -- that God can break the law of non-contradiction -- is meaningless.
When it comes to the specifics of God's revelation, and particularly the life, death, and resurrection of His Son, I think we're better off leaving the word "necessary" out of the discussion. These are the means by which God has chosen to offer us eternal life with Him; being chosen by God, they are (by logical necessity) the best means to achieve His will.
But we don't know God's will in full. We know it in part: the part that has been revealed to us. And we can, hesitatingly and humbly, infer more of it from the means chosen to fulfill it -- free will, for example, seems key.
So to say this or that event was "necessary" is to risk proscribing God's will in a way that God has not revealed it to be proscribed. That's a risky thing to do, not least because God's will can sometimes be identified with the Holy Spirit (St. Catherine of Siena did this often, the reasoning being that what one wills, one loves, and the love of God is the Holy Spirit even as the knowledge of God is the Son and the power of God is the Father). And we don't want to set limits on the Holy Spirit.