If you aren't doing anything extremely strenuous, then no it probably isn't necessary. But I've done a 100m and 200m sprint and me slamming the breaks at the end generally results in either me feeling faint or simply passing out.

I've never really paid attention to why a cool down was necesary, nor did any of my coaches require it. I just did it because after a good run your sweat glands are wide open and it feels uncomfortable to stand still in the humid, Florida sun.