The "magic" of strength training comes from a combination of a few factors, the most important of which is selecting the appropriate resistance level for each exercise and exerting the necessary amount of effort. Do you put in time at the office with a pal? Having a workout companion can be beneficial because it increases the likelihood that you will keep the routine up. But if you listen to the advice of someone who isn't properly trained, it's the same as if you let a friend "break your back" instead of going to a chiropractor to get your spine adjusted. Even if you have good intentions, putting your faith in someone who is not a trained professional could end up causing more problems than it solves.
Simply put, we do not have enough time to be "professional" in a lot of different areas. Time is saved when that responsibility is delegated to a trained professional. Additionally, it ensures that we will receive guidance that is more knowledgeable and experienced with regard to the training that will take place during the session as well as the design of the training program as a whole.