Inpainting lets you edit a single area of a generated room without touching the rest. You mask the region you want to change (the sofa, the rug, the artwork above the fireplace), give the model a new prompt for that area, and it fills in only the masked pixels in a way that blends with the surrounding context.
The technique is borrowed from image-restoration research — the same algorithms originally used to remove dust, scratches, or unwanted objects from photos. Apply it to room redesign and you get fine-grained control: try three different sofas without re-generating the whole room each time, or swap "wood-burning fireplace" for "minimal cast-iron stove" while keeping the kitchen behind it identical.
Inpainting matters because full-image regeneration is wasteful. If you like 90% of the result and want to fix one element, regenerating everything risks losing the parts you liked.