If you look at all the top athletes in the world, almost every single one has a coach and for good reason. If you are unsure of how to train for powerlifting or the correct form and technique for the 3 main lifts, the reason for a coach is fairly obvious, and that is to find someone with the experience and knowledge to help you optimize your training.
But many powerlifters have a very good working knowledge and possibly a strong educational background in the exercise sciences. But as someone who fits into that category, I can admit that many times we do not make the best decisions in regards to our own personal training programs. Maybe we tinker too much and question the plan. Maybe you are the person who needs accountability to not max out on any given day that you are feeling half-way decent. Maybe you have hit a wall and need the creative input and different ideas of another expert to figure out the solution to your strength plateau. Or maybe you just want to take the stress of programming off your shoulders and let someone else handle that burden. There are many reasons, but it all boils down to have a second set of eyes to guide you in the process and make the best decisions possible for you and your long term career in powerlifting.
Having a powerlifting coach increases the accountability to stick to the plan, it helps with having multiple creative inputs into your training (as the coach should be listening to athlete feedback and critiques as well), and a coach helps to catch things that many times the lifter overlooks. Its kind of like when you are dieting and its hard to notice day to day differences because the change is so slow. But your friend who hasn’t seen you in a month goes “Oh my gosh, you look completely different”. Having a coach to watch and critique your form is similar. You have seen the same movement over and over for so long, day in and day out, but having a coach who has no previous knowledge can analyze your form and possibly help point out things that you may have been overlooking.