I learned this a long time ago but it was a revelation!
It helped me become so much more efficient and effective with my time and also better able to jump into my repertoire after. Because you are not tired you are ready.
When you simply align your instrument instead of hammering away at it just to get "warm" you've already done half the job.
When it's aligned then you can sing the instrument warm and get the blood flowing and become more limber.
Same thing goes for the gym - align your body to the training you are about to do, don't tire yourself out before you've done anything that will bring you any results.