That is really not true even slightly. Uniforms are useful in most contexts. They can immediately inform everyone around them what that person is and what they should be doing. As far as militaries, uniforms were instituted to reduce killing your own people. Yeah, if you identify yourself as a soldier from faction X, faction Z will know to shoot at you, but also your allies from your own faction will know not to shoot at you. Even aircraft that shoot each other down at ranges that make it impossible to identify by sight send out identification signals so they know who is friend or foe.

As far as the assassin set, assuming these assassins are from Zamora, the assassins and their associated fearsome reputation are components of Zamora’s national defense. Having assassins walking the streets in identifiable uniforms furthers the purpose of their existence - scaring the poop out of their enemies and making them think twice before attacking Zamora.