During the attack on the first Death Star, various groups of rebel fighters enter the meridian trench (first Y-wings, then X-wings) in order to get to the (in)famous exhaust port. They enter in groups of three, but because the trench is narrow they could only fly in formation. Vader and his wing man then easily pick them off.
Since only one fighter could attack the exhaust port simultaneously (a small target, only one or two are torpedoes needed), wouldn't be better for another two fighters to get out of trench and attack Vader and his wing man directly in order to force them to break off their pursuit of the group leader who could then launch torpedoes without constantly looking back ? Anti-aircraft fire cannot be the reason, because it is almost the same inside the trench, and low above the surface .
Looks like only Han Solo understands this simple tactics, as he easily gets on the Tie Fighters tails and then blasts them, saving Luke