Beacuse my Friends, when footballer go off the pitch they are tired and hot. If they stop sit down without a coat they will cool down to quick and this could result in a cold. so they wear a coat to cool down nice and slowly.

It's to keep them warm as your body cools down rapidly when you stop exercising.

When your core gets hot, then the cool down can cause shivers, and muscle tightening

keeps you warm man in this uk weather.