Why Unity5 use offscreen framebuffer in forward rendering?Bug?

I used Adreno Profiler to check my game in a android device. It is compiled with unity 5.1.1f1 and GI is disabled.
I find the objects first render to a FBO. Then this FBO object render to screen and the shader is just copy pixel. Why to this? I think it is no use and waste memory and maybe

The vertex shader is :
void main()
{
gl_Position = vec4(vertex.xy, 0.0, 1.0);
texCoord = vertex.zw;
}

The Pixel shader is:
void main()
{
gl_FragColor = vec4(texture2D(tex, texCoord).rgb, 1.0);
}

I try to use other build/lighting/quality settings.,but no use: also render object to FBO.

So I check this in Unity4.6. It's right and directly render object to screen.

Can you tell me why?