Benefits of boiling meatballs before frying them
There's a few reasons for utilizing this method:


You'll end up with a juicier meatball, as it is cooked in liquid. 
It'll be rounder and more plump because it was cooked in a liquid. 
You'll be 100% sure that it was cooked thoroughly without being burned.


The reason for the pan browning is just a reverse sear - purely for color/crunchiness and perhaps some flavor from a browned meat and the butter.