The purpose, as set forth in the title to the link, is to let you access those functions without using a mouse.
There are letters for each tab. Note that Insert is N rather than I.
The first level is the tabs and the Quick Action Toolbar buttons.
If you select a tab, you then get more letters/numbers for the controls on that tab.
If you are a touch typist, these can be much faster than using the mouse / touchpad.
If you customize the Ribbon or QAT, those modifications also receive these shortcuts.
Here is the Standard Ribbon/QAT in Word 2019:
Here is my customized Ribbon & QAT:
00 deleteme 8.png
Note that the Ribbon Tabs and controls start with letters, the QAT starts with numbers. Customizations do not assume any of the letter shortcuts but QAT customization can and will rearrange shortcuts for the QAT.
First, there is no way to turn these off. So, ...
Second, you might as well learn some of them for your most frequent Ribbon clicks. You can always pick up with the mouse if you get lost.