This is something that the law says that someone (in this case, usually a local authority) must do, and that if they do not follow may result in legal challenge.

In relation to a public organisation such as your local council, duties are things that the law tells the organisation it must do. (Services that the council has a duty to arrange or provider are known as 'mandatory' services.) With regard to care and support, your council has a duty to assess your needs, arrange care and support if you are entitled to it, and provide you with information about what care and support is available.