It would cost the company next to nothing, and it would be a preferable alternative to the employee quitting due to stress or burnout.
For the employee it significantly lowers their risk of unemployment. They can either 1. go to a better company, or 2. return to their old company.
In the case of US companies, there's also the added benefit of the possibility of the employee and employer coming to a mutually beneficial arrangement on maintaining the employee's healthcare plan during the sabbatical.