It's a classic scene : in a gym setting with a number of people around (not all of them actively engaged in exercising), a person who wants to lift a weight/bar asks somebody to watch them, or spot them, and when they can't handle it anymore, the other person can safely lift away the weight onto the rack, so that the person exercising can stand up and walk away.
If there is no other person around, and they decide to lift the weight/bar anyway, and get unexpectedly exhausted, or any kind of unforeseen event happens (maybe one of the hinges break, or there's a mild earthquake, or a thousand other possibilities), they can easily die from having the heavy bar with its weights push down onto their chest or other body part near the head, killing or seriously injuring them.
If the setting is a private gym at home, with nobody ever coming there, it seems like a death trap. How can they dare to exercise there alone like that? I cannot imagine that every single person who has a little home gym always has somebody with them in their home.
Not being a gym-going person (I've actually never been in one outside of a school context), I've always wondered about this. Just the thought of ending up dying alone from your own weight/bar suffocating/crushing you sounds like a horrible way to die, and it doesn't seem far-fetched at all. After all, you may say to yourself: "I'm going to do 21 reps this time!", but you only have the energy for 20 reps, so the last one, intended to build up your strength further, instead causes you to die.
Maybe you'll answer that there are now some kinds of fancy security machines, but if so, I'm talking about all the years prior to this becoming common/standard.