I am not a Twitter expert, but I can tell you what causes me to unfollow someone. It's when they tweet something that either has no value or, worse yet, annoys me. I have unfollowed people because they used twitter to bludgeon followers with a particular political view. That's a quick way to lose half your followers unless you're preaching to a choir who follow you BECAUSE of your political views. I also get annoyed when people post what they had for breakfast, or just to say something like, "WOW, what a GREAT WEATHER we're having where I live!". Sure, that's a nice sentiment to express, but not enough to interrupt me or anyone else.
Then, of course, are the self-serving promotional tweets. That is an art. It's hard to promote yourself or your company without coming off sounding like you're doing it.
The purpose of a tweet is to enlighten, entertain, educate, humor, or in some small way, make the person reading it feel good. At least that's my opinion of how it should be used.