If they're at home they can play computer games, go out, watch a movie, etc...
I see being allowed to work from home as a sign of respect and confidence. Therefor I return that respect by not fucking with said opportunity.
Given the ability to schedule in chores or a bit of fun during breaks, my breaks become far more productive.
When I get stuck or something or am blocking on someone at work, I tend to browse hackernews, some forums, ... You end up with activities that draw you in more and more, potentially wasting hours without really having the refreshing effect you need breaks to have.
When I'm at home, I get up and vacuum a couple of rooms, clean the kitchen or maybe even skip out an hour to work out when the gym's calm to help clear my mind. I'm confident in saying my days spent working from home are by far the most productive of my entire week.
> If they're at home they can play computer games, go out, watch a movie, etc... Someone like that can just as well be expected to spend their entire days scrolling through cat pictures. Don't hire someone you think you can't trust working from home, they won't fare much better on-site.
Sometimes employees can be moved onto other temporary tasks while the other team completes their task/dependency but this causes the employees in the former team to lose focus - Making employees context-switch between too many projects carries a cost which may be higher than just allowing them to busy-wait from time to time.