There were three reasons why the Angel appeared to Joseph with this message. First, that a just man might not be led into an unjust action, with just intentions. Secondly, for the honour of the mother herself, for had she been put away, she could not have been free from evil suspicion among the unbelievers. Thirdly, that Joseph, understanding the holy conception, might keep himself from her with more care than before.
He did not appear to Joseph before the conception, that he should not think those things that Zacharias thought, nor suffer what he suffered in falling into the sin of unbelief concerning the conception of his wife in her old age. For it was yet more incredible that a virgin should conceive, than that a woman past the age should conceive.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).