Examples

White has a dangerous protected passed pawn on h7.
The promotion square is blocked by Black's king on h8.
White can attack the blocker with Nf7+.
After the forced reply Kg7, White can now promote with h8=Q+.
The new queen cannot be captured by the king, because attacking the blocker also has the advantage of protecting the promotion square. Therefore White can keep the new queen and will checkmate soon.

Sometimes attacking the blocker of a promotion square is worth a sacrifice, as in the following example:

Black has a passed pawn on g2, which is protected by the rook on g8.
Black can't promote immediately, because White's king on g1 is blocking the pawn.
Black can sacrifice a bishop and win by attacking the blocker with Bxf2+!
After the forced reply Kxf2, Black can promote to a queen with g1=Q+.
Black cannot capture the queen, because it is protected by the rook on g8, therefore Black has a winning material advantage.

Sometimes attacking the blocker can include other motifs:

White has a dangeours passed pawn on c7, which is blocked by Black's rook on c8.
Black just attacked White's rook with Kf7-e8, but that was a mistake.
White can win with the surprising Rd8+, attacking the blocker.
Now if Kf7, White wins with Rxh8 Rxh8 c8=Q.
And if Rxd8 White promotes with c8=Q, and if Black captures the new queen with Rxc8, White wins the rook on h8 with the skewer Rxc8+.

Visualization

When you are threatening a promotion, but the promotion square is protected, always think about ways to remove the defence.

Related combinations

There are various ways to remove the defence. In addition to attacking the blocker, other ways to remove the defence include deflection and capturing the defender.