During landings on the aircraft carrier, if you flew a consistently high approach (one ball/dot high), you would inevitably land short catching the first wire (not good) ---- as you would be underpowered with a high rate of descent and would not be able to "catch it" when you finally saw the "ball" (glide slope indicator) begin to descend in close.
Likewise, if you flew a slightly low approach, you would inevitably "bolter" (land long --- missing the wires) as you would be overpowered in close, too flat, and not be able to catch the rising glide slope at the last second.