We saw in the previous article the main line of the closed variation of the Morphy defence (3..a6),  and the advantages and disadvantages, we also briefly discussed why white generally prefers to play the prophylactic 9.h3 instead of the immediate 9.d4. Check the next diagram where white follows Yates Variation, 9.d4 and black responds with the very strong 9...Bg4, (Bogoljubow Variation)

So it should be clear by now why the move 9.h3 is considered a wise move, before d4.