Deep web is a gateway of Dark net and dark net is illegal and use mostly for crimes and it's unsafe. I don't think there is any good reason to use it. I guess we can find everything we are looking for in surface net.
Surface net is not safe for keeping government related documents, that's why deep net was made. Dark net is a just a part of deep net. Normal people also use deep net for staying secure. But before using deep net we should know how to be properly safe on it.