A power rack allows a trainee to train alone safely. For example, if you wanted to bench press in a power rack you could set the safety bars just above your chest and not have to worry about getting flattened like a pancake if you miss a rep. Same thing for barbell squats. Just position the safety bars at the depth that you want to squat, and now you can do worry free barbell squats. If you get pinned, just fall forward and crawl out. Of course, ideally, you will have a spotter at all times.