Why does Congress make laws that expire?

This year we saw a lot of drama around whether or not Congress would renew an expiring ruling onthe interest rates for student loans. Then a line from an article on the recent Colorado Shooting caught my eye:

The AR-15 rife carried by Holmes, a civilian semi-automatic version of the military M-16, would have been defined as a “semiautomatic assault weapon” under the Violent Crime Control and Law Enforcement Act of 1994 — which expired in 2004. “The type of ammunition magazine Holmes is accused of using was banned for new production under the old federal assault weapon ban.” Though once it expired, “gun manufacturers flooded the market with the type of high-capacity magazines Holmes used Friday.”

If we at one point thought it made sense to ban assault weapons for private ownership, why was that ban part of a law set to expire? why not make laws and then, when and if someone decides they no longer make sense, let them repeal them or make new laws? Expiration dates on sseem rather arbitrary and therefore nonsensical.

Can anyone explain this to me?