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Well... its not like you have a coach right next to you, checking your every action.
And also motivating you to work better.
Fitness trainer is a must if you want good results in the gym and not screw your health because of doing something really wrong.

Well I wouldn't say a must but if you don't have the time to do your own research, yes it may be.

Your own research + your personal coach is a must.
And also no one will disturb you while training when the coach is with you.