Uh, just maybe so that Mary wouldn't be in the dark about why she was gaining weight and appearing to be PREGNANT before marriage?
What the BIBLE states about an event like this:
... each man should have his own wife and each woman her own husband.
The husband should give to his wife her conjugal rights, and likewise the wife to her husband.
For the wife does not have authority over her own body, but the husband does.
Likewise the husband does not have authority over his own body, but the wife does.
Do not deprive one another, except perhaps by agreement for a limited time, that you may devote yourselves to prayer;
but then come together again, so that Satan may not tempt you because of your lack of self-control.
Tell yourself what you want; you can’t answer the question as to why he waited for her reply, or why she replied.