Can you explain in simple terms that if I buy a fire stick with kodi, does that means it automatically streams various films and music from Amazon or Netflix foc. Does it needs to be jail broken

I will never advice you to go for a preinstalled Kodi device. There is a lot of risk associated with it. And if your firestick is fully loaded which means it is jail broken, it is better you get rid of it. You can easily setup Kodi with 101 Best Kodi Krypton Addons for March 2018 Working List and streaming movies and TV shows.

Why would you buy kodi installed FireStick when you can do it yourself in a few minutes? I mean, OK, you don't have any problem to spend some additional money in buying Kodi preinstalled Fire Stick but what if you get problems later? It is always better to know how to install Kodi on Firestick as it will help you in future too. How to Install Kodi on FireStick

Yes if the Kodi already set up for u with a build and add-ons on the stick and Kodi u can watch sports movies and TV shows

You can install Kodi on firestick anyway. Just buy the normal Firestick and install kodi on firestick. And yes, you don't need to jailbreak the Firestick. You can install kodi on firestick without jailbreaking the device as Kodi is available for firestick (Android based). Once you installed the kodi, you will be able to watch all your favorite movie and tv shows. there are different methods to install kodi in firestick. You can choose any of them that suits you.

Sure Amazon along with Netflix jailtbreaking is not necessary. Having installed along with jailbreaking the device in my humble opinion Amazon is OTA updates are deliberately reconfiguring fire sticks knocking & blocking out all other apps