It gives them 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 tough for the enemy to get in and you can't get out.
Tracers work both ways.
The only thing more accurate than incoming fire is incoming friendly. Radios will fail as soon as you need fire support desperately. If you take more than your fair share of objectives, you will have more than your share to take. Both sides are convinced they are about to lose - they are both right. Professionals are predictable - but the world is full of amateurs.