Why do you need a SIM card and a ESIM card on a IPhone, what is the benefit of it?

You don't need both. You can use one or the other. A sim card, wether physical or eSim, holds your service plan settings and phone number.

It is entirely up to you wether you want to use one or both. Also, it will depend on whether or not your service provider supports eSim, not all do.

Using eSim makes it easier to setup, as you don't require a physical card to insert, and setup can be done through a QR code, email message or your carrier's App.

Also, having a secondary sim, whether physical or eSim, will let you have a second phone number and service. Many people for example, now have their personal and office cell phone numbers on the same device, and take calls for both things.