As Will says, one function of the repetition in the examples you give is that they can serve as a refrain, which other people can join in and feel that they are carrying or engaging with the song. (E.g. "The answer, my friend...." or " Long time passing.... ").

In ballads, there's often a set of several answers to riddles, attempts at some feat, etc, and the repeated words emphasise that the struggle and determination required to achieve the right outcome.

Such lines can also serve to give the singer time to remember what comes next, but perhaps that's just my personal take on it.

Marje