Pretty to look at, indicate that you completed an event, express some degree of individuality, a way to show off that you got to a portrait reward before others and I think some are based on staff which is cool. They don’t change anything in game. There is a Santa one and a Tour de France one which I think look good but don’t have as I started playing after they were obtainable. Mainly they help you recognise who is messaging such as on group threads, although a lot of people have the hooded portrait so it doesn’t always work