Oh!, you people always want answers to your questions, and we are so busy trying to be clever*. Sometimes the answer just gets lost in the confusion….
A roaster oven might be more appropriately called a steamer oven, because it keeps in a lot more of the cooking moisture than does a conventional oven. Of roasting a turkey in a roaster oven, one manufacturer poetically says, "The constant dripping of condensation from the cover of the roaster prevents the turkey from browning but also produces a turkey that is moist and juicy."
Unless you are intentionally steaming vegetables or dumplings in your roaster oven, you do not need to add water. If you are intentionally steaming vegetables or dumplings in your roaster oven, one manufacturer recommends you do so in a metal colander suspended above the liquid.
*Well, what we think of as clever, some people just see as rude….