When you're traveling and don't want to plan weeks ahead, your options with physical SIMs typically were either

1) special roaming-oriented SIM cards from your local country (which were still a massive hassle and had to be ordered days in advance to allow for delivery),

2) getting a SIM in the destination country (hassle and might not always be possible)

3) very expensive offerings from your own provider, if they even had something better than the aforementioned car-per-GB rates.

Now, you can get an eSIM from an MVNO that's from a third country, before travel or from the airport/hotel WiFi, usually also bypassing any bureaucratic bullshit that the destination country imposes on getting a local SIM.

And these "travel eSIMs" are often cheaper than local offerings even for use in your home country.