It’s for little survival servers so that you can see that your friend found diamonds and now he can share them with you You can also enable or disable the types of ore you want to announce in the config.
Also, you should have this be per-block-id. Instead of manually defining which blocks can be broken w/ notification, allowing any block to be defined would be handy.
A server owner could also use it to keep track of how frequently a player runs into iron/diamond/emeralds in order to weed out xrayers and other ne’er-do-wells!