Why use KODI on a streeming box over default interface

Hi! I’m just learning about KODI. I hope you don’t mind this very introductory post. I hope someone can help. My objective for looking into KODI is to see if there is a way that I can legally increase our television viewing options and perhaps eliminate cable TV. We don’t watch much live TV. When we do, we’ll just switch to the local channels. We are usually watching Netflix (movies and TV series). We spend some time on Amazon Prime. It is usually whatever we can get in the subscription or for free. Here is my big question. A friend recommended that I purchase a “MI Box” for $69. He then told me to get an app for the MI Box called SMPC KODI build. If you purchase a media streaming box with an interface on it, why would you want to use KODI? What does KODI have that is not in the interface for the media center box? From what I understand, KODI is a media center interface. To get movies and television series, you need to get apps to do that for you. Some are free and the others are subscription-based. From what I understand, if I were to do this, I would have two media center interfaces to choose from; the one that came with the box and KODI. Is this correct? What are the advantages? Would I be able to increase my viewing options. Thank you for putting up with this introductory post.

Terry