During a firefight, communication is difficult. Squad (9-11 troops) and team (4-5 troops) leaders have to show their guys where to go, and where to shoot. If there's a lot of noise, or if silence is important, hand signals will be used. But pointing out a hard-to-see target (sniper, where several enemy are taking cover) is often difficult. Especially since you want to do this fast, as the "target" is shooting back. Some battlefield leaders have found that a solution for this is to keep a small (20 round) magazine full of tracer (they leave a trail of fire) rounds in their rifle. Keeping their weapon on single shot (rather than full automatic), enables them to fire a few rounds at where they want the rest of their men to fire. The leader can pull the small mag out, insert a larger (30 round) one, and join in the shooting, as needed.
This technique has proved very useful in Afghanistan, where the longer distances encountered, mean that there are more places the enemy shooters can be. A squad or team leader gets the job partly by having better "situational awareness" (a sense of what is going on around them), and that includes being the first to spot where the enemy troops are. A few tracer rounds in the right direction, and everyone knows where to shoot.
On the downside, firing those tracers also attracts enemy attention, whether or not they know about who is using the tracers. That's another risk battlefield leaders have to deal with.