With a free-trial VPN, you might get only a basic version that might also be limited in speed or security restrictions. What is more, you might not even have full access to the support. All that is made to force you to get the full version and pay.
With the money-back guarantee, you get complete access to the VPN service with an option to cancel your subscription.
The money-back guarantee gives you the full potential of the VPN with an option to quit in case you are not satisfied.