That thing on the door is a handle. A knob would let you adjust how far the door opens, how much it resists being opened, whether or not it shuts itself (and how quickly) and how far you have to turn the handle to get it to start opening. Clearly most doors work just fine without knobs.
Tech knob discussion, how about a nice boring dictionary answer.
1 a*:* a rounded protuberance *:* lump b*:* a small rounded ornament or handle
2*:* a rounded usually isolated hill or mountain This seems that a knob doesn't have to be useful. Brian
