Maybe it could be for the psychological benefit of the person wearing the uniform? Like, if youve got a post scarcity utopian society where people dont need to work, but youve got some job that needs doing that can be a bit dangerous or tedious at times and therefore might be inherently unattractive to many, like ship's crew, and you cant just pay people a bunch to do it because you've done away with money, one incentive you might have to offer is social status and a sense of personal achievement. In which case, you might use bright flashy displays of rank, because it gives the person that has attained it a bright flashy reminder of "hey look at you, you've achieved this fancy rank that everyone that didnt join the fleet or didnt get as far as you yet doesnt have", and gives any regular civilian that sees you a blatant reminder of "hey, this could be you, if you're willing to put in the work for it"