Recently, I asked around about the reasons for using chosen icons (subset) and not stocks (every icon from one stock). Personally, I think the best reason for that is the speed of our page. A lower weight of an asset which we need to download can play a big role for a user. If the website has a lot of heavy assets, the user can get frustrated, especially, when using poor mobile internet connection. If the internet speed is not sufficient he will see some invisible icons or squares in the place where we should see the icon.
Speed is one of the most important factors of your