Any injury can be difficult during the healing process, but some of them take long enough a person needs therapy. It might seem ridiculous that a person has been independent and able to walk or write for decades, but an injury can make it impossible for them to do those things again once the cast comes off. A physical therapist is trained to help them with learning t move again, and it can be a long and painful process.
Walking seems to be particularly difficult for those who have sustained an injury, and much of the issue seems to be they feel they already know how to walk. Many of them know and remember how to do it, but their muscles may not respond correctly to their mental commands. The job of the therapist in this case is to help sync their thoughts with their actions. It can be frustrating the first few times, but daily practice will soon have the patient moving on their own feet.
Fine muscle control is important in many areas of modern life, and writing is one example. Most people today learned how to do it at a young age, but a hand or arm badly damaged might not work they way they expect once their injury has healed. Being able to exercise it properly to regain control can take patience, and a therapist can help them. Teaching them how to retrain their own hand might seem ludicrous, but it could be a necessity. The control will eventually return, but they will need to work hard to regain it.
There are many times when injuries are said to be debilitating, and that facet of it often shows up after the healing is nearly complete. Loss of control over muscles due to lack of use is one of the main ways people are adversely affected. Learning how to move once more and control those muscles can take a lot of work, but the majority of times it is well worth the effort.