There are many people with Bronchestasis condition that use the Breather.
It can be helpful to strengthen the muscles of respiration for those who need to cough up mucous frequently - stronger muscles make for more efficient coughing. Exhale against resistance with the Breather is also helpful in mobilizing mucous in the airways. This gets the mucous up to be coughed out more readily.
Whether moving ahead with the Breather or not, I would recommend checking out the Huff Cough Maneuver as well as it can be so helpful for those that need to get mucous up regularly: