Light in a room is not a single layer.
It is not. The light you need to move around the room is not the light you want when you sit on the sofa. Asking one ceiling for both does not make the room better.
What is a layer?
A layer is a separate job light does in the room. GE Lighting names three: ambient, task, accent. Interior practice adds a fourth: decorative.
Ambient light lets you move around the room. The Illuminating Engineering Society calls it a spread environmental light. Task light lights a desk or a book. Accent pulls a painting or a wall texture forward. Decorative light, like a chandelier, shows itself as well as lighting the room.
The light you need to move around the room is not the light you want when you sit on the sofa.
Why does it not all come from the ceiling?
The ceiling is the most common way to build ambient light. It is not the only way. The same job can be done with light spread on a wall. When ceiling light is at full power, the room looks the same everywhere. Moving around is easy; there is no separate soft layer for sitting on the sofa.
Changing the colour of the ceiling bulb does not change that job. The light still arrives from above. A strip stuck to a wall only lights one surface. It does not wrap the whole room the way ambient light does.
Where else can ambient light come from?
From a corner, toward a wall. You see the light on the wall, not the source. The room may be dimmer than it is under ceiling light. Sitting on the sofa is still easy.
That ambient light is our job. It does not replace a desk lamp or a picture light. It does not replace the ambient light in the ceiling either. Ambiance is the feeling that light builds in the room. The definition is in what is ambiance?.
What happens to the other lights?
A lamp for a book and a source for the table stay in place. Opening the ambient light does not switch them off. If you leave every light fully on at once, the layering is gone. The room looks like one light source again.
What light does to alertness is a separate subject. It is in does light actually affect our mood?.