H2 priests: Why, when, how?

[Maraxus]

Blessing has an area of 5 per level
Banishment has an Area of 2+2 per level and damage 3 + 2 per level.

(And 6*7=42 is more then twice as good then 4*5=20, isn't it? )

But anyway, the main point should be, that in turns you recruit your nation's priests, you don't recruit your nation's mages.

[analytic_kernel]

Communions can be another reason for building H2 units, since you can boost a H2 communion master to H3 or H4 with a smaller supporting cast than a H1 communion master. This can allow Divine Blessing on more armies than the one with your prophet, if you don't have any other H3 commanders available. Also, acts a force multiplier as Maraxus notes.