I bring one dressy black skirt and one pair of black palazzo pants - with several dressy blouses in various colors. One pair of dressy sandals.
One formal dress suffices for the "formal" night and the sandals go with that too.
I love to clean up for dinner - wouldn't dream of going out in jeans and tee shirt. I don't think people need to wear evening gowns and tuxes for dinner but it doesn't take much to freshen up and get out of jeans/shorts unless you're grabbing a burger at the pool or going to the informal buffet.
People behave differently when they're in nice attire and it lends a more graceful atmosphere. But to each their own!