They make it for those of us who don't want to spend 3-fold its price on a breathable jacket.
Both types will make you sweat, anyone who thinks differantly is just kidding themselves or have been conditioned by snazzy advertising. Breathable jackets only let the sweat evaporate into the atmosphere, and at a fairly slow rate mind you.
Personally, I don't mind getting wet. When I get to work I change into my work clothes anyhow and my riding clothes have plenty of time to dry.

My body sweats to regulate its heat;
cold rain causes hypothermia;
I ride with a non-breathable rain jacket. And I saved about a yard on the cost.
So tell me please, why is it "just a bad idea".