The metaphor of Gatsby “com[ing] alive” and being “delivered suddenly from the womb of his purposeless splendor” has a very strong meaning in the overall message of the novel - Why?

This is Fitzgerald using figurative language to great effect. Gatsby wanted to be born again into Daisy's presence. The fantasy of life with Daisy was Gatsby's sole reason for living. Of course that is all it was: a fantasy. There was no reality or sense of real purpose behind it.