One of the reasons is because the designer typically chooses three colors and repeats various shades of those colors throughout the main living spaces in the home. Usually all three of the colors will appear in every room, even if only showing up in a few accessories. This creates a cohesive pallet and ensures that all of the rooms relate and feel like they belong in the same house.
Think about the big picture to give a home flow. That does not mean that every wall in the main living space needs to be the same. Weave the same colors throughout the main spaces, but make the dominant color in one room an accent in another. You’ll be surprised at how different the rooms will look, yet how easily they work together. You can choose to use one of the colors by painting an accent wall, or use the colors in furniture, rugs, window treatments, art or accessories. Find patterned textiles that have the same colors, or create window treatments using solid fabrics and color blocking. Mix it up so that each room has similar window treatments, but not exactly the same.