For Oskar's grandmother and grandfather the doorknob was the gate from the "nothing" world to the "something" world.
Doorknobs are, like doors, a universal symbol for the gateway to something new. However, unlike doors, doorknobs are what you actually touch and interact with to get to the other side. So, in essence, doorknobs symbolize the way to get from one point in your life to another: in Oskar's case, with each doorknob he comes closer to accepting and understanding his father's death. Oddly enough, Oskar is opening doors both metaphorically and physically. In opening each of the Blacks' doors (physically, since he is actually knocking on doors to get in, etc.), he is progressing along in his journey to find the object his father's key unlocks, which is his metaphorical doorknob because the journey is an elongated version of turning a knob to another part of his life. Also, having affected each Black he has visited, Oskar is also "turning doorknobs" in other people's lives, as well as his own.
Another reason doorknobs are significant is because they are the medium through which Oskar's Grandfather watches Oskar, through the keyholes.
One of the most significant themes of the book is doornobs because they represent a part of Oskar's life that he needs to pass through in order to move foward in his life. On the picture pages throughout the novel the doors appear different as if they are trying to represent Oskar's advancement toward his goal. On one page the door is locked as if he needs to find a key, while on another it doesn't have a keyhole so he could cross it at any time, but he is not yet prepared for what awaits him on the other side.
With Oskar's grand father using rooms in their home as "something" places and "nothing" places. It is a large symbol of not wanting to be too close to reality. The element of the door knob means a wide variety of things mainly for the grand father it is a symbol for those places chosen to either enter into or leave unopened. When Oskar's grand father takes pictures of the door knobs before he leaves it is his own personal way to think of his life and with Oskar's grand mother in their home together.
I think that doorknobs are important becxause they were what Oskars grandfather made his hands rough.