Put yourself in this scenario..

You have worked out for years and gotten results.  Since you love the fitness world, you decide to become a personal trainer without really knowing what it entails.  The job sounds exciting and you have friends that are already doing it.  How hard can it be?  I mean all you have to do is write workouts and motivate others.  You already give your friends advice and drag them to the gym

After some research you decide on the certification that's right for you and complete the course.  Most of the information was second nature to you because you've always loved to read about working out.

Through your connections, or job classifieds, you get hired on by a Fitness club!

Congratulations!

Now the club starts pairing you with clients.  They range from 15-75 and most have musculoskeletal issues that need special attention.  You panic.  All of your research to this point has been to improve your own workouts.  You have no idea how to program for beginners let alone beginners in a different demographic who have injuries and low motivation.