Fighter pilots started using tracers to direct their fire, but the characteristics of a tracer round is different than a ball round. They found that 80% of ball rounds missed their targets when tracers rounds were apparently hitting the target.
Fighter pilots also noted that when you fired using tracers the enemy anti-aircraft fire could more easily find the aircraft.
For some time fighter pilots had the last rounds on their belt ammo loaded with all tracers so they would know they were running out of ammo...guess who also figured that out...