I'll take a guess here on this since I work in an office on a computer all day.
Possibly the thinking is that you're a little busy doing work but you know in the next couple of minutes that you need to use your phone for something. While you're busy scanning your monitor you reach over to where you know your phone is and touch the button to unlock the phone without looking at the phone because your eyes are on your monitor.
But you're still a little busy taking care of what you need to do. You finish that and then reach over to pick up your phone to do what it was you were going to do with it knowing that because you used TouchID it's already unlocked and you can directly proceed.
Having to juggle more than one thing at once at work this is a scenario I can see happening.
My guess on this anyway.