Yes, we may already know the stuff they teach, but will we
really do the exercises on our own, at home, with all the
distractions? 
Also, joining a class makes me push myself harder, since everyone
is doing it, or if I can't do it, I'll work towards being able to
do as well as the others. Then there are others who enjoy the
socialising/mutual encouragement in a class. So, joining a class
can be helpful for some ppl. 
