Why to use "May" before using "May God bless you"?
Solution 1:
See meaning 4 of may at dictionary.com
(used to express wish or prayer): May you live to an old age.
It follows the same grammatical pattern as let (and is almost a synonym).
Let their children grow up happy!
May their children grow up happy!
The usage of may in this sense is not restricted to prayers, although one could say that it's formal, if not pompous, in modern usage.
Solution 2:
Putting "May" first changes the sentence from a command to a request. Rather than instructing God to bless someone, the speaker is rather hoping that God will do so.