They're to close down the interstate in the event of bad weather, accidents, chemical spills, etc.
Here in my area of Illinois we don't have them but go over the state boarder of Wisconsin and they have them -
https://www.google.com/maps/@42.560...7l2GPU5jEg9fBxJw!2e0!7i16384!8i8192?entry=ttu
I've seen the northbound gates being down a few years back as there was a truck turned over that closed down all of the northbound lanes.
Cheers