Why do we crumple paper?

I don't know why people do this, but probably this is a habbit. I'm trying allways to flat things before throwing it away.

Because otherwise being light, the paper would come out of the bin and when we crumple it, its centre of mass shifts to a larger place and hence, it does not come out, it stays over there easily or else if you ever drop a paer as such in the bin, it might come out with any gust of wind.

I guess it would be because if it is a paper I don't want someone to look into the trash to see what was on it, I would crumple it. That way the only way they could read it would be to reach in and uncrumple it. Most people are curious by nature, so if the paper was open, they would go ahead and peek at it...but if it is crumpled, most won't waste their time to uncrumple it.