Hi there,
I'm now a MSK physio but I do have about 2 years experience having worked in ITU/resp wards and at the National Spinal Injury Unit where we used supportive/assisted coughs on a daily basis. In spinal it was used to help clear secretions in patients who didn't have the capacity to carry out an effective cough independently (mainly due to the level of their injury) and subsequently reduce the chance of a chest infection- a potentially huge problem for higher level spinal patients. In general physio I have used it with patient's who are having difficulty with coughing secondary to pain. You could also teach them how to do it independently themselves with a towel. The aim with both general and spinal patients, as far as I am aware, is to clear secretions and reduce the chance of chest infection.
I hope this has been some help.