Honestly I feel vpns are just kind of like gym memberships, it's not expected for everyone who gets one to use it every day, even though they could.
The only real use case I can foresee this for is for people who might use a VPN for a few hours, a few times a month. With that kind of usage pattern $10 (The min topup value by the look of it) could last you a fair few months so works out cheaper than some of the other mainstream VPN providers who offer a flat fee service.
If I may use analogy to describe UpVPN - its like buying Milk - you pay upfront you bring it home consume it and go to grocery store and buy more.
UpVPN is an option in spectrum of VPN providers. Only you can determine based on your usage if this option makes sense for you.
What UpVPN does provide (unlimited devices without subscription and your never expiring balance stays if you come back months later) other providers do not. And vice-versa UpVPN for its pricing model does not provide unlimited usage.