Counselors have trained for the job; they understand behavioral patterns, thought processes, can identity causes to mental health symptoms etc better than a friend or family member. Whilst the latter will support you because they love you, the former will support you because they can (hopefully) steer you in a direction which will help you overcome whatever obstacles you are facing.
As for their reasoning behind it, it could be many things. It could be purely for financial gain, for example, but more often than not it's because they genuinely care for others and want to do some good in the world. Yes, their previous experiences may also be another reason for their career choice.