Double Naught Spy
Sus Venator
Why to do that? As I understand the tracer also gives out the shooters position and besides, ...
So basically, you are asking why anyone would ever use tracers since they have the potential to give away one's position.
The notion of giving away one's position sort of assumes that the enemy can't discern where the fire is coming from. Given that snipers don't usually use tracer because being clandestine is so critical, especially when shooting, the enemy will have a pretty good idea where you and your unit are if you are firing on the enemy.
Loading a tracer a few rounds from the end works fine for semi-auto fire. It lets the soldier know he is close to the end and that he needs to be considering a reload asap. Depending on targets available to him, the amount of time before he must reload after the tracer will depend on how much shooting he is doing.
In daylight, tracers are very hard to see from the side or from in front of the bullet although the tracer will be visible to the shooter if he is looking for it. It just won't be that bright in daylight.