They do this for one of mostly two reasons. Both are to keep cars at bay from passing to buy time. Either they need to slow traffic if there is debris in the road and a dot or state cop are pulling it off the highway ahead such as a tire, or something that fell out of a pickup, or second reason they're buying time for a road crew to get on an exit down to start laying cones for roadwork. I usually speed past this if it's a dot foreman or a tilcon truck.