The average length of characters per reply is there to help users know how serious a particular user is.

Most time-wasters often send very short messages to many people, rather than focusing on just a few users that meet their criteria.

You may be disappointed when you correspond with such users, since the messages you receive from them is likely to contain short vague statements. More often than not, they do not answer the questions you ask them.

So if you find a user that has a habit of sending very short messages, such a user may be a time-waster not worth communicating with.

On the other hand, a user whose average characters per reply is say a thousand or more, may suggest a spammer who sends the same very long message to a lot of users.