I don't think it's "obvious". If the company's completely closed, nothing at all is happening, no work is getting done, no one is available to cover emergencies. A better comparison might be to look at 1/3 of the company being gone in July, August and September, which, if you work with people to plan things just a bit, means that things keep running during those months. Also, what starts to happen here (Italy) in practice, is that in July not a lot gets done because everything is going to shut down soon, so no one starts anything new (no new projects, no hires). Things already start to slow down in June. September is reasonably productive, but it takes a bit to get things turning over again.

Staying open conserves momentum and provides some continuity.