It's been around a few seasons. A player can be placed on there and it frees up a roster spot, but they stay with the team and can do all team activities or seek outside help for mechanical issues.
Normally, pitchers are placed on it to limit innings while still letting them practice, sit on the bench, ect. Sometimes hitters are placed on it instead of the IL too, for the same as above if they have a minor injury or are working on a new swing.