Though the question is asking "why" someone would want to do it, I will answer it in a different way.
Consider there are 20 faculty members (senior professors, professors, associate professors, senior lecturers, lecturers, non-teaching staffs) in a department. Someone must control and coordinate in/regarding the department with the university higher ups.
Usually, many universities follow a nomination procedure where usually Senior Professors or Professors would be voted to become the Head of the Department (this system is followed in my university). Please note that there is no compulsion to become the head. If a person has been nominated by the majority and (s)he is not willing to become the HoD, then the next nominated person in the order would be chosen.
Otherwise, the university admin such as director, vice-chancellor, or deans would designate one professor to be the head of the department.
Usually, the assignment is not for life time, rather it is assigned for 2-3 years (depends on the university rules and regulation). And, there is no harm in doing something different from usual teaching and research stuff. It is not completely true that the HoD can't do research along with administrative loads, actually, he can. But, might not be that productive.
However, the quantity of teaching load for the head completely depends on the university norms.