Passport validity rule of 6 months (3 months in limited countries) are enforced by countries for visitors (travelers/tourists) to minimize overstaying in a foreign country beyond passport validity. Typical tourist/visit visas have 6 months validity so they want to ensure that Passport are still valid when people return back. Airlines do not let passenger board their plane with passport expiring within 6 months because if the immigration of the other country does not allow the person to enter that country because of passport expiry issues, the airlines get fined and the airlines have to bring back that person to the originating country at their own cost. This is applicable to people going to a foreign country not to own home country.