Not having to physically put a SIM in your phone.

It has e at the start.

I usually try different custom ROMs and have to format my phone frequently. Will that eSIM survive that format or not ?

Being able to get an app from the app store and activate a service from it in a few minutes, no need to wait for a physical card to ship.

Vice versa, you can cancel your service on that app and swap to a new one in a few minutes as well.