Other than offside trap, why play high line?

I play mostly defense. Have played many years. Played as CB on high school team and did pretty well. BUT the one thing I never quite got comfortable with or really understood was why defenses often press up high, like to basically the midway line many times. I understand doing that if you're going for the offside trap, but even when not doing that it seems that defenses feel they have to "push up" but without actual knowledge of why. Oftentimes this leaves a huge gap between the defense and...

Other than offside trap, why play high line?