Why was Jesus virginally conceived? Admittedly, the question is somewhat speculative. However, I think the Bible expects Christians to reflect on the theological significance of the virgin birth, so this is more than idle speculation.
1. Let's begin with secular explanations. On one version, the virgin birth is based on pagan exemplars. But there are familiar problems with that allegation:
i) The alleged parallels aren't comparable upon closer scrutiny. For instance, the women may not be virgins. Or conception involves copulation between male gods and human women.
ii) The pagan stories are too far removed in time, place, and genre to be exemplars.
iii) It would be repugnant to Matthew's Jewish audience. That would be counterproductive to his aim.
2. On another secular explanation, the virgin birth is a cover story of a prenuptial scandal. On one version, Mary and Joseph jumped the gun. But there are problems with that explanation:
i) According to Mosaic Law, premarital sex was not a capital offense. The punishment was a shotgun wedding.
ii) If, moreover, Mary and Joseph were already betrothed, then fornication is a technicality. After all, it took a formal "divorce" to dissolve a betrothal. From what I've read, there was no consensus on whether it was illicit for betrothed couples to exercise that privilege.
iii) Although there was no legal double standard, I suspect there was a cultural double standard. How much stigma, if any, would attach to Joseph? Surely a fair number of single Jewish men were sexually active. That's why Proverbs warns against young men frequently with prostitutes. Likewise, what got David into hot water wasn't promiscuity, but adultery, and betrayal (of a soldier under his command). If this was a prenuptial scandal, it would only be scandalous for Mary, not Joseph.
iv) Since there'd been no scientific way to prove paternity, Joseph could simply accuse Mary of sleeping with another man and wash his hands of the matter.
v) Unwed motherhood was hardly a unique occurrence in 1C Judaism. Why would anyone find the Virgin Birth a plausible cover story?
vi) A variation on the secular explanation is that Mary was pregnant by a man other than Joseph. If so, it's inexplicable why Joseph would consent to marry her. That would be culturally demeaning to Joseph.
vii) Since the secular explanation regards the account as fictional, it would be simpler for Matthew to deny Mary's out-of-wedlock pregnancy by narrating that she became pregnant after Mary and Joseph tied the knot. If, according to the secular explanation, Matthew is guilty of fabrication, why not a fabrication that eliminates any grounds for suspicion?
A possible objection is that Matthew couldn't get away with that because there were witnesses who knew Mary was an unwed mother. If so, that generates a dilemma for the secular explanation. Those who treat the virgin birth narrative as fictional or mythological date Matthew to c. 80-100. They think it was written by an anonymous author with no historical connection to Jesus or his relatives. But in that event, how would anyone in Matthew's audience be in a position to correct his account if he denied her prenuptial pregnancy? Mary and Joseph weren't famous at the time of her pregnancy. Only a handful of people would know when she became pregnant. And on liberal dating, that was about a century (give or take) before the Gospel was written.
vii) Finally, this isn't the only Biblical example of a miraculous conception. Unless all other examples are cover stories for prenuptial scandals, why assume that must be the explanation in this case? Why single out a prenuptial scandal in this particular instance?
2. Let's shift to theological explanations. One rationale is that if Jesus had a biological father, then he'd inherit original sin.
One problem with that rationale is that mainstream Reformed theology affirms the immediate rather than mediate imputation of Adam's sin. It's something everyone gets direct from Adam, by divine imputation, and not from your parents, or your father in particular.
3. On the face of it, a divine Incarnation doesn't necessitate a virginal conception. On the one hand, Jesus doesn't require a human father to have a divine father. Those are separable. They operate on different levels.
On the other hand, if he can be human without a biological father, he can be human without a biological mother. After all, Adam and Eve were human sans parentage. He could be human with two parents, one parent, or no parent. Different miracles.
4. Another rationale is that a miraculous conception is a divine sign that there's something very special about this person. And that's undoubtedly true as far as it goes.
5. In addition, although his divine sonship doesn't automatically preclude a biological father, that omission draws attention to his divine sonship. Even though these operate at different levels, yet because it's normally necessary for humans to have biological fathers, if someone doesn't, the follow-up question is to ask who takes up the slack? Who fills that role?
6. Finally, it might seem initially odd that Christ's claim to Davidic ancestry is merely legal rather than biological. Isn't that rather roundabout? Doesn't that seem to weaken the connection? The claim would appear to be stronger if Joseph was his biological father rather than stepfather.
But if you think about it, the way God actually arranged it is more subtle and powerful. How does one become a king? One way is through inheritance. Passed down from father to son.
Yet that's not how David became king of Israel. Jesse was not a king. David was a commoner.
Rather, God directly elevated David to the throne. And David's coronation employs adoptive language: "I will be to him a father, and he shall be to me a son" (2 Sam 7:14). God is like David's stepfather.
By the same token, Jesus isn't the rightful heir due to his human paternity. Rather, he's enthroned by God himself.
In bypassing genetic lineage, the virgin birth creates a partial parallel between the kingship of David and the Davidic kingship of Christ. Jesus is heir to the Davidic throne, not in virtue of his physical pedigree; rather, God directly installs him as king just as God did in David's case. So there's a type/antitype parallel.
7. Moreover, in typological escalation, Jesus is God's Son in a way that David is not. Jesus is God's Son by nature, and not adoption.
In fact it creates a chiasmic relation:
A. Jesse is David's ontological father
B. God is David's stepfather
B. Joseph is Jesus' stepfather
A. God is Jesus' ontological Father
One way to contrast two things is by comparing two things. Their similarities make the dissimilarities stick out.