White has a simple back rank mate with Qe8. Just for the sake of analysis, though, would Ra8 work as well? The a8 square is unguarded, but consider Black’s interpositions. If he drops his knight onto d8, the rook’s path to Black’s king is blocked—and if White plays RxN, Black has QxR. Instead of RxN White could then try Qe8, but this loses his queen: when Black moved his knight he unmasked a line from his bishop to e8, so the square no longer is available. Thus Ra8 doesn’t work. Notice that the order of operations can be crucial. If Black replies to Ra8+ by interposing his queen rather than a knight on d8, he loses; White drops his queen to e8, Black replies QxQ, and then White has RxQ#.