A pigtail gas tube is a "fix" used for rifles with cycling problems . Usually, these are experienced by those which use 11.5, 10.5 or 7.5 barrel lengths. The added length of the tube (because in spirals around the barrel) in a pigtail minimizes, if not totally eliminates, problems of bolt bounce or short stroking. [It has something to do with the amount of gas pressure blown back. I don't specifically understand the technical reason. I just know it helps.]
In your case, if your rifle does not suffer from these malfuctions, then you do not need to have this gas system.
If you have full auto capabilities, then as mentioned, the pigtail will reduce your cycle rate making your rifle more controllable in full auto fire. (But a brake is more the cure to any problem of controllability.) But, if you are only have semi-auto, then again, you do not need a cyclic rate reducer.
IMO .... other more learned and technical members may correct me if im wrong.