Why is it so important to KEEP a pawn from promoting if you have a piece situated so that when the pawn promotes your next move can be to capture it? Is this a matter of tempo? Frankly, once the pawn has reached the seventh rank why go through so much effort just to protect it? I guess I am new at this.
Usually, there's a piece behind that pawn that supports its promotion. Then you're eager to stop the promotion otherwise you'll have to give up your piece for the promoted queen.
If the pawn isn't supported, then the reason for blockade might be prevention of some tactics that could be related to a sacrifice of that pawn.
Aron Nimzowitsch once said:
The passed Pawn is a criminal, who should be kept under lock and key.
Firstly, if you can keep the pawn from promoting without losing any minor pieces, why not try to? Pawns that are close to being promoted (usually passed pawns) are often defended (e.g by a rook on the same file), and this means that if they are allowed to be promoted, it usually results in the piece used to capture the promoted pawn being captured itself by the defender of the pawn. This is clearly not good. In addition to this, all pawns with the high potential to queen in the future (I.e passed pawns) could become a threat if they are not dealt with later on in the game. Even if they do not appear to be a threat because, as you said, they can be captured if they are promoted, in the future the piece(s) that are defending against this pawn promoting can never be moved from their position (rank for rooks, diagonals for bishops) lest the pawn is allowed to queen. In addition to this, your opponent can put even more reinforcements on the passed pawn if you do it try to defend it, improving its chances of being promoted, which is obviously not good!
Just controlling the queening square might not be sufficient if your opponent can devise some back rank tactic using the pawn for support. Then when a piece of his, say a rook or the queen, enters the last rank onto a square supported by the pawn, capturing that piece will result in recapture by the pawn with promotion. And if his piece isn't captured, it can then support the pawn promotion on the next move by now contesting the queening square that you controlled.