Doors seem simple, but they control how you can use a room. A door placed in the middle of a wall, for example, might make it almost impossible to put a bed, sofa or wardrobe there.
When deciding on a new door position, it helps to sketch the room with likely furniture layouts. Where could the bed go? Where would you put a wardrobe or desk? Does the door swing clash with those pieces? Would shifting the opening a little to one side create a usable wall for storage?
In living rooms, door positions affect seating arrangements and TV placement. In bedrooms, they affect privacy and how comfortable the room feels when you enter.
Thinking about furniture early avoids that depressing feeling later of “nothing fits right in here” after you’ve already spent money opening or shifting a doorway.