'Holy God' why that apostrophe?

What is it about that apostrophe?

Below two examples of how not to use it. And even those, whose first language is not English, are making the mistake.

The bicyle ad appears on a wall in Tavira, Portugal.

And then in the current issue of the Dominican Rosary Newsletter. Why is the plural of Hail Mary Hail Mary's?

Sort of nonsensical, indeed, all nonsensical.