I love having an extra Plying Bobbin. I can fill the first plying bobbin with my newly plied yarn and leave it on the bobbin while I'm spinning new singles. Once they are ready, I can load up the 2nd plying bobbin and leave it sitting. By the time I'm ready to ply again, the yarn on the first plied bobbin has been there long enough to "set the twist", and that yarn is ready to skein.

And then..... there's always a need for a 2nd plying bobbin because I've switched to spinning a different colour ...