Now, let's say you accidentally unfollowed somebody or you unfollowed somebody because you lost interest in their content or didn't converse much, but later decided to follow them again. You don't want to annoy them with an alert or deal with, because some people are like this, them inquiring about it for whatever reason. (I personally wouldn't care, but to each their own.) This is where an option to silently follow somebody, despite their alert settings, could come in handy.
This could be added in the form of a confirmation box coming up after you click follow. "Would you like to follow this user without them receiving an alert?" or some form of.