I knew about social media buttons because I have been blocking them ever since they began but I was not aware of how widespread or rather ubiquitous they have

I knew about social media buttons because I have been blocking them ever since they began but I was not aware of how widespread or rather ubiquitous they have become. What is the point of them? Doesn't facebook already have this functionality on their own page? Those share buttons seem to me like something that should be a browser plugin not part of a website.