“This smelly, filthy garment must be made out of some special fabric in order to have withstood the foulness of the bridgekeeper's body. Oddly enough, it has plasma burns and scorch marks all over it, as if these weapons were used against it... to no effect. You have NO idea why it's purple.”— in-game description, Fallout 2
As one would note it has the exact level of protection as a set of combat armor but with half the weight and no bonus to Radiation resistance, making it perfect to use before getting a set of power armor and, later, by one of your companions.
The macro that verifies if the Chosen One is wearing the Vault jumpsuit doesn't include the Bridgekeeper's robes in the list of items it checks against. So, these robes are not counted as concealing the jumpsuit.[verified]