Family, friends, and mentors are frequently excellent resources, in part, because they have developed relationships with you over time, know you well, and have your best interests at heart.
The flipside to this positive is that the advice they give may not be impartial.
A coach is, usually, someone who you don't know who brings fresh eyes, ears, and perspectives to your issues. They have immediate access to information and tools you need to fix the problems, that would take a great deal of time and energy to amass on your own. Their #1 goal is to help you attain the outcomes you desire.
Just as musicians, visual artists, dancers and other areas of artistic endeavor participate in continuing education (lessons and masterclasses) with great artist and teachers, business people engage coaches and work with mentors for fresh perspectives and to gain new information which helps to prevent stagnation in their personal and professional growth.