I do that with spare analog inputs - so that if I want to hook a new sensor up, I can see at a glance if it's working without having to drill into the controller tags.
I've also done it on occasion where I just want to be able to see the value of that tag on a particular rung for ease of troubleshooting. An example might be a high level alarm - let's say that somewhere else in the code you have a bit set that warns of a high level. That bit is then used to turn on a siren. It might be helpful to someone looking at the code to be able to see not only that the siren is being turned on because of a high level, but also what that level actually is. So, MOV the level value into itself, and it's visible right there.
Of course, there's the point that this information should be displayed clearly on a HMI or SCADA system - and that's true, but for the purposes of an example it gives you the idea of why it might be done.