Why Do People Who Ride Motorcycles Wear Heavy Jackets Even In Summertime? [VIDEO]

Why do people who ride motorcycles wear heavy jackets even in Summertime? I see them on open roads like I 10 or I 49 and even roadways like Congress, Johnston and Ambassador Caffery.  At first I thought it had something to do with speed, but I don't think that's it at all.

It is law in Louisiana to wear a helmet, but is it law to wear a jacket even in the dead heat of Summer?