[snowwrestler]

Same as Facebook--maintain the relationship but reduce the noise. You can only DM with followers for example.

[eterm]

Technically, you can only DM people who follow you. Whether you follow them is irrelevant.

Twitter is a pub sub model, it doesn't really make sense to want to sub but not sub. Twitter follows aren't like friendships at all, they're a "I want to see your content" marker.

[marblar]

Yes but the fact that they are publicly visible makes them serve a secondary purpose.