is multiplier?but it is useless.
It partly depends on your materials. Multiplier and decay settings give you control of the lights power, also global ambient light color has a strong effect on all lighting and materials in the scene.
You can also control how the surface material on meshes reacts to the light. In fixed function, thats what speculat and emissive, ambient etc are for. If you want extra light power to get a cheap bloom effect something like a 2x multiplier blendmode combined with lightmaps to modulate works really well giving you something a little like HDR/bloom with richer shadow areas, and brighter glowing specular lighting and saturation.
If your using shaders, the lighting is controlled almost entirely in the shader code and you have to code exactly what features you want on a per material/mesh basis. Ambient, and fixed function settings have little or no effect on lighting unless you support them in your shader programs.