Sunset+Clause

What is a sunset clause, and what is its purpose?
The formal legal definition is "//A statutory provision providing that a particular agency, benefit, or law will expire on a particular date, unless it is reauthorized by the legislature."// But why would legislation that passes in both the House and the Senate want to create a sunset clause into law? Since the only way to continue the legislation is for Congress to then continue to examine the institution created and thus change and evolve to the new needs of modern day society, a sunset clause is a good check from one session of Congress to another. // // By providing an option for more debate and changes to a bill, a sunset clause encourages debate from those who oppose or greatly support a piece of legislation. In this way, a sunset clause is another affective check for Congress upon the Executive branch or even on itself, and promotes democracy in the law making process.