It depends on what you are sanding.
If you need to remove material (straight line isn't straight or curve isn't smooth) I use 80 or even 60 grit to speed things up.
If you are sanding just prior to finishing, 120 0r 220 usually works well.
If you are polishing a finish to get a deeper, glossier look, I use brown paper sack and then maybe a coat of paste wax (and rub it hard) to make it silky smooth to the touch.
It all counts as "sanding" to me.
Rog