I would take off a jacket, because it would hinder arm movement, but not a t-shirt....course I'm not a fighter.....
i saw an idiot do that one time at a party......he ended up getting thrown in a bonfire.... funny as fuck.....
I did it for 3 different reasons
1st- scare tactic to intimidate the person I was about to fight,
2nd- so they didn't have anything to grab hold of to control (pulling shirt over your head ....)
3rd- so your shirt does not get torn ,bloody/dirty.