As was explained to me in simple terms.
The packs on the -800/900 push out a constant temperature air which is a little cooler than crew and passengers would normally find comfortable. Via the mix manifold this air is fed to the three zones. As this air is cool it needs warming up to reach a comfortable level which is where the (hot) trim air plays it's part. The -400 has essentially the same system. In my FCOM page 2.31.7 shows a diagram.
I was told it is easier and more efficient for these packs to run at a constant output temperature, and then add warm air as needed, as opposed to providing air that is too warm and adding cold air, or modulating to provide conditioned air.
The -300/500/700 series (and probably the -100/200/600) use a different pack which modulates the output temperature within the pack itself to provide a comfortable level of conditioned air.
I am sure an engineer will come along shortly and be able to provide a more detailed answer.
Hope that helps.