Structured training with a coach isn't for everyone... It depends on your goals and level of experience. If you want to race for fun, know how to train and ride well, then no, you don't need a coach.
Also if you're elite and already very good, a coach can only do so much, you can't expect miracle.
If you feel you've got a lot left to learn about, training, physiology, tactics, diet and lifestyle, then you have the most to gain.
Goal selection is key to the whole process too: Do you want to win elite races, turn pro, loose weight, become healthier, enjoy riding more?
I wrote a post about this a while back when I was asked this question for the first time:
http://grimpeurheureux.com/do-you-need-a-cycling-coach/