Why water go out to the drain of my car?

By "drain hose" I think you mean the exhaust pipe. If you see water coming out of there, it's usually the dew (moisture in the air) that has collected in the car's muffler overnight, and that spills out once the car is being turned on and driven. This is a good question, I bet a lot of people don't know this, but they have seen it. But don't worry, it's nothing bad, just water coming out of the pipe. It's not coming from the engine or anything.

Dude, WTF kind of vehicle is it? Many makes and models will run the AC automatically if the defrost or heat/defrost position is set for the air distribution. That's why the evaporator drain tube still drips water. Where you getting worried your car needed to take a piss?!