When to 'Add/remove reserve or youth team player to first team training'

[ryandormer]

In what circumstances, if ever, would you use this:

I considered using it because I wanted to add a reserve team player to a mentoring unit, while keeping him in the reserves. However, it didn't work. I assume the player needs to actually be in the first team squad in order to be part of a mentoring unit. 

Which leads me to wonder why you would ever use this feature. Surely if you wanted a player to undergo first team training, he would be in the first team!

Do any of you ever use it, and if so, what for?