Well after b8=Q black can capture this new queen with the bishop Bxb8. After this black is winning with 2 extra bishops. After Ra8+ instead black must play Bf8 to block the check (if Bb8 Rxb8+ Be8 Rxe8#) and now you can go b8=Q which is crushing.
Why is this move better?
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Thank you very much for the explanation! I see the logic behind it now. 
Hello,
I am seriously brand new to chess and am practicing with Lucas Chess. I am trying to figure out why the tutor says that Rook to A8 is better than my choice of Pawn to B8. Can someone give me some insight into why that is?
Here is the chart:
Thanks!!
Jason