Well one reason is to know what kinds of rights you do and do not have so then you know how to defend yourself if you feel that your rights have been violated. This also includes knowing what other people's rights are and making sure that you don't violate theirs.
Another thing is to know how the government works so then you know realistically what you can do to make a difference as well as knowing who and what has power/control over the things you care about. That way, you will be able to make wiser political choices.
There are definitely more reasons than these two however these are the ones I think are the most important.