Stock image dump for running Magisk necessary?

In the offitical installation instructions for Magisk it says: “However, you MUST have a stock boot image dump beforehand, and you also have to be able to flash the patched boot image, either through fastboot/download mode or ODIN” *What’s the rationale for having the stock boot image dump beforehand?* If the rationale is to have a backup when something goes wrong, then strictly having a stock boot image dump is not necessary for running Magisk. Furthermore, it is unclear to me how to create that dump. PS: My plan is to root the Xiaomi A2, to be able to directly interface with USB OTG devices from within Termux . The Xiaomi A2 is an Android One device.