The first reason is that machines are safer than free weights. People can go to the gym by themselves and not need to be spotted by a second person. It eliminates the risk of having the risk of the weights being to heavy for a person to hold and them falling unto them and injuring them.
The second reason I found out is that in the facilities
around here people are for the most part beginners in the gym. I have been in the gym working out and watched people doing there exercises with free weights. They do things that are risky to their bodies and may in the future cause serious injury. This is a good reason to eliminate the problem of safety by using mostly machines.
The third reason is for the trainers. This is so the trainers can keep watch over the people lifting and make sure it is not too heavy for them and if it is they can quickly change the amount of pressure to ensure safety.
I have seen three types of machine with the options of how to switch the weights. The first and oldest here is the actual weight with a bar pushed through to choose how much you lift. The second is a dial with six options of pressure. Finally the third is pressure gauge with controls on the floor that you can press with your foot to change it.