Your first question was, what was in it for both parties, and I answered: Mary got a master of the house and Joseph got a homemaker. Your next question is, why not just cohabit, as relatives sometimes do. Well, since Joseph fathering a child with Mary was not exactly out of the realm of the possible, a sexual union between them would have been at least rumored, and therefore improper. Where did you get the idea that Mary was Joseph's niece? Surely whatever relation they had, if any, did not preclude marirage, and by the same token would have precluded cohabitation without marriage.
everything [Annalex] pointed out was found in this document
Simply because the document is rather comprehensive in presenting one version of events, that is consistent with the scripture, and specifically consistent with Luke 1:34. But is is a document of uncertain provenance, not canonical, and so is but a historical evidence dating in 2 century. The official teaching is expressed by the late Pope in the link I gave you in 6678, and he merely states that it is a mystery why Mary was betrothed while committed to celibacy.
The mystery is why you believe such baloney when the Scriptures clearly indicate otherwise.
It was suggested as a possibility by another poster, but I cannot remember who it was. I certainly do not believe that it is correct.