most newer recreational divers do not have a way to secure their primary second stage and would either just let it drag behind them, keep it in their hand, or hope it stays on their right shoulder.
many advanced divers (as mentioned above) will tie a bolt snap to the second stage so it can be clipped to a dring. but these same divers typically use a different regulator set up than a new ow diver.
in the past, my wife and i used to put two octo holders on the bcd. one on the left and one on the right (yes we have our octo on the left). so whenever the primary second stage was not in use, it was secured to an octo holder. usually we used this on the boat when gearing up so it didn't fall and it was safe, neat, and right where we needed it.
this is a simple, cheap way to do it. so if you had a long swim while using your snorkel you could use it for that as well.