Every choir/chorus I have belonged to has always started rehearsal with warming up. I have found that when I am running late for choir and do not warm up in the car, my voice is nowhere near what it can be.

As I understand it, it doesn’t improve your voice, but rather stretches (loosens) your vocal cords – basically it is tuning the muscles to be exercised when you sing.

I suppose that could lead to improved singing.

It’s very similar, I believe, to the idea of doing stretches before you being your workout. You want to loosen up.