FIRST...let me say that PROFESSIONAL LIABILITY is often muddled with GENERAL LIABILITY.
Professional liability insurance is insurance that protects business professionals, such as doctors, lawyers, and accountants, from negligence and other related claims their clients may file. Professionals with expertise in a certain area need this type of policy, as general liability insurance does not cover negligence, misrepresentation, malpractice, and other business errors or omissions that may arise.
Everyone makes mistakes...but even if the insured DIDN'T make a mistake, the cost of defense agains an accusation can be costly and the professional liability insurance pays for that defense.
Generally, it may come under different names , depending upon the field the client is in, but it provides insurance for those who "don't do their job correctly" or are ACCUSED of doing so.