Interiors photography is a subset of architectural photography and similarly, requires specialized gear, planning, and skill to translate a space from 3D to a 2D representation. Similar to exteriors photography, ambient light and the right time of day are important to best capture a space. However, interiors photography has additional challenges from color casts and a greater exposure range.
Lighting is used to complement the interior space, and not overwhelm it. Supplemental lighting helps with color, exposure, and adding depth. My use of flash is subtle to help address the interior challenges and to match the design intent of the space.
Colors casts are caused by reflections off of colored surfaces, environmental films on windows, ambient light mixed with various interior lighting which range from magenta to green, and yellows to red. With the broad use of LED and fluorescent lighting, accurate color is difficult for interiors photography without using supplemental lighting.
Supplemental lighting is used to add the color range that may be lost due to the artificial lighting. You can see how I handled the colors for this shoot in my Behind the Image post. The exposure range is usually larger for interior spaces due to darker inside versus the bright outside, so without supplemental lighting windows will have glare around them, and/or the interior will be too dark.
The key is to add just enough light to mimic what our eyes see, and maintain the original feel of the space. The third reason to add light to a space is to add light and shadow to better represent the 3D space in 2D imagery. Shadows and highlights add depth to an image, and sometimes supplemental lighting is needed to achieve the desired effect.