Having just finished going through 20 or so Qigong postures at home, I can only say that a class session is preferable.
I am sure our instructor James would agree. He would probably explain that the group experience increases our qi or life energy and multiplies its positive effect.
I'm not really sure what that means.
What I do know is that in the classroom surrounded by other people all doing "Double Hands Support the Heavens", I am not seeing dustbunnies or a drooping bunch of flowers, and I am not thinking about unfinished Christmas cards or overdue library books.
Besides, a friend and I go out to lunch afterward. "Double Hands Support the Coffee Cup" is very qi enhancing.