Why Text Message

Most of the reasons James, Dustin and others have mentioned.

Sometimes it would be disruptive to take or make a call, sometimes I don't feel like talking on the phone, sometimes I just have something to say or information to share, etc.

Think about it. If I'm meeting a friend at a bar and will be 10 minutes late, does that require a phone call?

A quick, "be there in 10" is faster and less intrusive.