Our company requires us to wear uniforms during empty legs and refer to a psychological experiment that says it makes you approach the job at hand with a more professional attitude instead of complacancy and boredom.
in the US, you get away with formal casual in slacks and a polo, but internationally it gets you further looking official with bars on the shoulder and does ease things when navigating thru airports. Of the more stupid rules was Lisabon airport, where the manager decided a pilot must look like a pilot and wear the jacket with bars too.. otherwise you cant get to the ramp.