The first benefit of a free barbell squat compared to all the other alternatives is that it stimulates a full body workout. A barbell squat requires the ability to balance the weight high up on your upper back which engages the core as it sways left and right slightly when you squat. Compared to using a dumbbell, it does not recruit so much of the forearm so you will be able to pile on more weights and focus on activating more major muscle groups which will not fatigue as quickly as your forearm. Even when you are squatting on the smith rack or using the leg press machine, there is a higher tendency to hurt your back and your kneecaps due to the restrictions of space and mobility. The very thing that is supposed to make machines safer has actually harmed more people rather than help them.
The next benefit of the free barbell squat is that it allows you to grow bigger all over. Have you seen people who just focus on training just one part of their body and look lopsided? I have also seen people who just train up to the point that they are unable to train their lower back due to the sheer pressure pressing down on the disc column of their spine. Simply putting on 5 kilograms of mass solely on your upper body will increase the stress on your back, hips and kneecaps by over 400% due to the activity you engage in besides working out. Having a weaker back for an extended amount of time will just lead to an earlier retirement from the gym, not just for your workout but also permanently due to the injuries you may sustain.
The 3rd great tip in regards to free barbell squats is the ability to trigger a lot of the hormones responsible for growth. This is because the more muscles you engage during a workout, the higher amount of naturally occurring growth hormones will be released. This leads to more growth if it is coupled with a proper diet and sufficient rest. This is even more efficient to grow your muscles if you are able to actively squeeze the muscles required when you are coming up from the lowest point of your squat. Actively engaging the muscles will produce more microscopic tears in your muscles which will then provide a higher potential to grow bigger.
Next, a free barbell squat always has progression. Have you seen someone getting stuck at doing a workout at the gym? Getting stuck at the same weight and workout will stall your growth and could even potentially backtrack it. Having a progressive workout is important because it gives you a confirmation that your workout, diet and rest are all efficient to trigger the growth. When your workout produces the results you seek, it will strengthen the habit of going to the gym and doing the workout again. This works even better than having a fitness trainer to push you every time you work out.
The 5th reason why free weight barbell squat is good is because it works your legs and buttocks better than any fitness equipment out there. Getting a barbell, squat rack and weights as a set is cheaper compared to any leg workout machine on the market. Why would you pay so much more for something that cost 2-3 times more and only have the flexibility to benefit you less?