We are talking incubation period here not contagious period. The incubation period is the time it takes for the first symptoms to appear after the exposure to the virus.
So you could have been exposed to the virus, have the virus and carry it but not show any symptoms for that time. According to the CDC that time is between 2 and 21 days. However according to the WHO it is between 2 and 42 days. While most do show symptoms earlier it is not true for everyone. This makes a big difference if you are going to monitor potential patients. They are playing the odds with a 21 day quarantine as they realize a 42 day quarantine is not realistic.
We are told that patients are not contagious until they exhibit symptoms. Once a patient exhibits symptoms and if they survive, 70% do not, they can remain contagious through their body fluids for up to three months.