If you have ever used what is so-called "international data roaming" while you were oversee, you must have experienced the "Bill Shock" that is meant to be an unexpectedly huge amount of bill for a bit of Internet access. Sometimes you are not even aware of when or how you used it. Who wants to see the bill after a fun journey?
Public WiFi "hotspots" in places like cafés, airports, hotels and libraries are convenient but unlike your home computer you don't know what security these networks have or who else may be connected.
No need to worry about weather or not your rented-car has navigation system, if you take a Wi-Fi modem to a trip with you.