By booking through the cruise line, you have the safety and security of knowing the tour provider will get you back to the ship on time. Usually you will experience everthing that is described in the overview. If you are in a foreign speaking country/port, then you may also have more motive for booking on board. If you just want to take a 1/2 day tour, go to a beach or go to a particular site, then you can opt for a taxi/van tour probably cheaper than on the ship. Keep you tours to half day mornings so you have plenty of time to return to the ship for lunch then venture into the port for exploring.