How many times did you step through a doorway today? Most likely, you weren’t keeping count, but if there is a door installed in that doorway, it got swung back and forth many times during the day. It might feel like doors swing effortlessly on their hinges, but in reality, they are subject to quite substantial shear force. If the door is not installed properly, it could be damaged over time.
Having a door installed properly starts with the doorframe. It needs to be square and firmly attached to the surrounding wall. If your doorframe is not in good shape, the door might be damaged, not close right or in extreme cases, fall right off.
The screws that attach your door to the frame need to be long enough to connect well.
If your door is painted, it should cover it well on all 6 sides. Otherwise it might absorb moisture and swell, which could lead to it not fitting properly in the frame.