This is because, from a functionalist perspective, a sick individual is not a productive member of society. The patterns of sickness are often caused by persistent pain which helps to support their attitude of not wanting to take positive action to get better.
Therefore this deviance needs to be policed, which is the role of the medical profession. Generally, Parsons argued that the best way to understand illness sociologically is to view it as a form of deviance which disturbs the social function of the society.