The second spelling shortens the word from two syllables to one, making it more musical and/or easier to rhyme with ('prayer' rhymes with fewer.words (e.g. layer) than pray'r (e.g. air,fair, chair, stair, Claire, there, spare, dare)).

pray'r is nonstandard. I'd imagine that you either saw it in a text from a millenial or in a pre-20th century poem. If it was the first: who knows why kids write things the way they do? If it was the second, the apostrophe was used to show that the word should be pronounced as 1 syllable instead of 2 to fit the rhythm of that particular poem. However, in modern pronunciation it is already just 1 syllable by default, so don't worry about it.

Nothing, other than a concatenation of prayers. I've never seen it used and a search turns up very little. You should not use this spelling.

Singers and poets are free to invent new spelling. The rest of us should stick to the dictionary .