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Helmholtz resonators are often used on header pipes like that to help smooth out the power curve. Since the pressure waves of the exhaust travel at nearly the speed of sound (basically a fixed velocity), and the exhaust pipe is a fixed length, certain RPM ranges have imperfect harmonics which cause dips in power due to a loss in scavenging - meaning that the timing is the only variable. By adding a resonance chamber to the header pipe, it can help to decrease the intensity of the pressure waves, and smooth out the power curve. Not really meant for more power, but better overall power.
It is also often used on intake pipe systems to help eliminate "bad" resonance wave point and smooth out airflow. Or even simply used to reduce resonance noise.
Harmonics and Resonance waves are one of my weird hobbies lol.
In the case of this pipe, it may actually serve some benefit, but more than likely nothing to write home about. There are a lot of variables at play that can completely change the locations, size, shape, inlet area and length of the chamber, that unless it is specifically designed and built for a particular engine, with a particular cam profile, to a specific pipe length (Node and anode relationship) , and then desired affected RPM range, it is likely not going to do a whole lot of anything useful. It may alter the sound a bit though, which is possibly why it is there...or simply for looks.
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