Dressing appropriately is short-hand for "I speak your language. I understand cultural norms and can adhere to them. I won't embarrass you in front of your peer

Dressing appropriately is short-hand for "I speak your language. I understand cultural norms and can adhere to them. I won't embarrass you in front of your peers and clients." If you don't dress appropriately, they will have to find out all those items through discussions with you, and it definitely adds a hurdle to any initial discussions.