It can be for other things too though, like crashes. The longer the slowdown, more likely, the larger the debris (stoves, washing machines, etc) or debris strewn out over a long distance. I've seen two buckets worth of short rebar pieces scattered across a half mile of interstate, that took a long time to clear - any one of those pieces when flipped up by traffic would easily penetrate a windshield - it took a long slowdown to clear.

It also could be for moving a crash out of traffic and off the highway, getting disabled vehicles out of traffic, or some other road hazard. In order to initiate a slowdown though, there has to be a good reason, something in traffic that is a significant hazard, or something coming into or out of the roadway that would be a hazard for traffic without warning. All slowdowns are recorded with dispatch, including the reason for the slowdown.

Slowdowns prevent a lot of crashes.