It gives the enemy other people to shoot at. If the enemy is in range so are you. Friendly fire isn't. Anything you do can get you shot including doing nothing. Make it too tough for the enemy to get in and you can't get out.
Tracers work both ways.
The only thing more accurate then incoming enemy fire is incoming friendly fire. Radios will fail as soon as you need something desperately. When both sides are convinced that they are about to lose they are both right. Professionals are predictable, but the world is full of amateurs. All weather close support doesn't work in bad weather.