I mean, even if most people don't get sick enough to miss work for 2 weeks a year, 10 sick days a year is a pretty common number to give an employee, either by statute or as part of the benefits package. Even if they don't get used completely every year, most businesses are budgeting for their employees to be non-productive due to illness for two weeks each year.

And obviously colds and flu aren't the only way people get sick and miss work, but if you could cut those two weeks per year to one week per year by having three weeks of quarantine every four years, that's a productivity win before you include the more predictable "nothing happens in February this year", the economic value of the lives saved, and the savings on healthcare spending on the people who don't get sick.