I'm confused at the scenario you are presenting. So in this case, you travel out of the EU, but are installing a taxi app from a non-eu country, but the app is only available in the EU?
Maybe my example wasn't the best since Uber is practically in all countries. It has however competitors which also are available in many countries and could be available as apps in this EU only store
Let's get to a more down to earth example then: you break your phone while abroad, you can't buy a new one and reinstall old apps anymore.
Or another one: you forgot to install an application available only on this alternative store before leaving (it's not that crazy, see F-Droid on Android where apps are not always available on the Play Store), you can't install it anymore since you aren't physically in the EU anymore.
