Self-government (the equality of rulers and ruled, government of the people by the people instead of government of the people by an elite sprung from the people) is an important value because it gives people control over their own lives. Most people want to be masters of their own lives and want to be involved in the creation or transformation of the conditions and circumstances of their lives.
These conditions and circumstances include, of course, legislation. You have self-control and self-government only when the laws you have to obey are laws that you agree with; “quod omnes tangit, ab omnibus approbetur”, what concerns all has to be approved by all. And the best way to have this kind of approval is to allow the people to make the laws themselves or at least to allow them to participate in the process of legislation, for example by way of the election of the legislators.