Why would you want multiple render targets?

Question

In d3d11, you can bind multiple render targets ID3D11DeviceContext::OMSetRenderTargets. But why would you want to do this?

Answer 1

When multiple render targets are bound, they can each be written to individually by the pixel shader -- it isn't (necessarily) the case that all render targets will get the same image. You could write only the red component into one output, only the blue into another, et cetera.

This is used when implementing deferred rendering for example: position, normal, color, et cetera information are each written to their own render target.

Answer 2

The added benefit to using multiple render targets, as in deferred rendering like Josh mentioned, is that you send the scene's geometry to the vertex shader only once and apply it to different pixel shader outputs, instead of re-sending the geometry for every output you need. This saves a lot of computation time for the GPU. The trade-off for faster GPU performance is higher consumption of frame buffer memory.