Phrases – Why Say Forever and Ever When Forever is Enough?

According to the OED, it dates back to the 1526 Tyndale Bible, listing 1 Timothy 1:17. However, it also occurs in other parts of that bible. Here's a list:

So then vnto god, kynge everlastynge, immortall, invisible, and wyse only, be honoure and prayse for ever and ever.
_{1 Timothy 1:17}

And the Lorde shall delivre me from all evyll doynge and shall kepe me vnto his hevenly kyngdome. To who be prayse for ever and ever. Amen.
_{2 Timothy 2:18}

Grace be with you and peace from God the father and from oure Lorde Iesus Christ which gave him selfe for oure synnes to deliver vs from this present evyll worlde thorow the will of God oure father to whom be prayse for ever and ever. Amen.
_{Galatians 1:3-5}

And when those beestes gave glory and honour and thankes to him that sat on the seate which lyveth for ever and ever
_{Revelations 4:9}

"Forever and ever" is one translation of "εἰς τοὺς αἰῶνας τῶν αἰώνων", which is more literally translated as "into the age of the ages". See here for more info.

By repeating an element (sometimes going further and adding more "and ever"s), it adds emphasis.