You can travel to a different country, find your data is crap because reasons -- either they're not roaming well (hello edge on a cheap out of country provider), or you've travelled to Switzerland and data now costs 7.13eur/mb. (yeah, a car/Gb)
So you pay for an ESIM from an MNVO, get a qr code, install, and you're good to go in 5 minutes from any network connection. Nothing physical required. And your normal number is still there for incoming calls/texts, because all the ESIM phones I've seen have at least two slots (or one + physical sim)
Of course, you've got to have an unlocked phone. But you wanted that anyway.