Not all companies have a HRM department, so many of the duties fall on the employees direct supervisor. At some point in your career you will deal with the supervision of others. If you do not, you will be supervised by someone else and will need to know your rights. This is why you will need to be familiar with the responsibilities and roles the HRM manager plays. Because it deals with people.
People are all different.
The way you deal with one person may not be suitable for the way you deal with another. What makes one person happy and motivated may have the opposite effect on someone else. Managers have many challenges they face on the job everyday. I believe that one of the greatest challenges faced is employee job satisfaction, or rather the lack of it, job dissatisfaction. Dissatisfaction on the job can be a huge obstacle to a supervisor.