Before backing under a trailer, why should you check that the trailer height is properly set relative to the tractor's fifth wheel?

Correct answer to question 90 (Combination Vehicles): A trailer set too high or too low can prevent proper coupling or damage the fifth wheel

Why

If the trailer sits too high, the fifth wheel may not catch the kingpin properly, and if too low, the tractor frame can strike and damage the trailer during the approach, so checking height first avoids both problems.