The Citizenship system of rights and privileges of people belonging to this nation over foreigners is a powerful tool to attract new immigrants to your empire, some of which are ready to work for you right away. Besides that, new recruits are conscripted to speed up the improvement of the land around your cities.
This is a starting (level 1) Policy, and a great way to save yourself some building time in the beginning of the game. Not only you get a free Worker (which usually takes 10+ turns to build at the beginning of the game!), but it, and all future Workers, become more effective at their main occupation - building improvements! Consider, however, the state of your technological research at that time. If the land and resources around your starting city demand more advanced techs to be developed, then you will be wasting a Policy and your Worker won't be able to help you much!
Citizenship is the policy of a nation that acknowledges that a certain group of people, usually based upon their race or tribe or where they were born, are members of that nation, and as such, deserve a certain amount of protection as well as some rights and privileges, above and beyond those accorded to foreigners. For example, a citizen can often move around the country freely, while a non-citizen may be subject to deportation or imprisonment if discovered by the authorities.