With Apple using esim technology in iPhone XS there is much discussion about it. Lets take a look at what esim is? For that first lets understand what a sim card is. Sim card is the small card that has a chip that carries your identification to the carrier.
With esim instead of a physical card and a sim slot device has a inbuilt sim technology . A small chip in the device is used to identify the device as a subscriber to the carrier. This chip is programmable over air. To use this technology you need to use a QR code or a carrier app that supports esim.
Changing carries with esim is also easy.You do not need to wait for new sim card to arrive for a few days. With no physical sim card no hassle of size of sim card and sim tray adaptors.
Advantages that a manufacturer gets is that this technology makes the device more compact by saving space for physical sim and getting away with sim tray makes the device more dust and water resistant.