I am assuming it is to insure they do not have any illness that can spread to other dogs or wildlife. but I would imagine if you had a health certificate this quarantine would not be needed? but maybe it is for certain countries.
The UK requires most pets coming from different countries to spend time in quarantine to stop the introduction / spread of rabies and diseases. There is no rabies in the UK and therefore resident pets are not vaccinated against it. There are quarantine kennels where the pets stay for up to 6 months so they can be monitored.
Some pets are able to get exemption from quarantine via the Pet Passport scheme.