The virgin birth was not originally part of the story. It was tacked on because in those days, to be accepted as a god or son of a god or of any divine origin, you must have a virgin birth. So if they hadn't invented that story few gentiles would've been willing to even consider him.

The first two chapters of Luke are a blatantly obvious addition. In Matthew it's not quite so simple, but I believe it's a translation, and the translators made numerous linguistic improvements. In a few surviving editions based on the original Hebrew, the first two chapters do indeed have a different writing style than the rest of the book. But I think they were already intact when the Greek translator picked it up, who was able to beautify and harmonize the text as during translation, therefore replacing the styles of the original authors.

Not all early Christian sects believed in a virgin birth. Many believed he was simply born naturally from his biological parents. Others said he wasn't born at all, and either was a ghost or an allegory for what was going on in the spiritual world at that time in a way humans could understand it. Paul and whoever wrote John may have held this view. And both of them have the theme of being tasked through revelation to evangelize.