There are really many good reasons to work the horses in hand.
I would like to tell you when I use work in hand and for what reason
When autumn and thus the rain comes, I can have periods where I only work the horses in walk. The arena can simply get too wet to do much more than walk work. Then I often do work in hand with the horses.
I also work the horses in hand if I'd like to have trained all four horses, but don't have that much time. It's quick to put on a bridle and then go out and get them to work over their back for 4-10 minutes.