As others have said... in the military, in the case of machine guns, to walk your stream of fire onto the target at distance (although we always thought "the machine-gunner's life expectancy is short enough without providing even clearer visual evidence of where we are sitting).
In the civilian world - there really is not tactical necessity of them - but they are fun - especially for folks who have never seen them - or have never been to a night shoot.
Part of "gun culture" that folks tend to understate is that it can and should be fun at times. You can both be both "safe and serious" - and have a great time. It can be a rush to shoot something bigger, or faster (auto), or louder, or brighter - and that is a good thing. Tracers, when used in an appropriate environment, fall into that space...