Besides giving people time to unwind to relieve stress, and spend time with friends and family.
It also helps with reducing unemployment.
With 155 Million Americans employed, increasing the vacation the people get by an average of 2 weeks, would create the need for 2 million people to be employed to cover the work during those vacations.
I think mandatory vacations have an economic spreading of employment factor also.
It does cost business more money, so it hurts in the 'race to the bottom' competition between corporations where they each try to pay people as little as possible to get the cheapest prices, but with it being enacted with everyone, and tariffs on goods shipped in by countries without those policies, it would balance out the competitive loss. Or instead of higher prices it could cut into shareholder returns.
(Small business might need an exception so single person businesses would not have to close for 4weeks, but besides that it has many advantages.)
Here is an article on American vacation time.
http://www.prospect.org/cs/articles?article=time_off_for_the_overworked_americanAmerican workers receive the least vacation time among wealthy industrial nations. And it is no thanks to the U.S. government --127 other countries in the world have a vacation law.At least in context of vacation time, USA seems to be winning the 'race to the bottom'. The wrong race to run in my view point.
Although if people had some time off work, they also might spend more time thinking about what kind of world they want to live in, and the way some things work and could be fixed, something many do not want to have happen.