When you make engage in free weights exercise-making use of free-moving objects, it forces our ‘stabiliser’ muscles to balance the weight we are holding.
When you make use machines, instead, they do pretty much all of the work for you.
This shows why a barbell squat is so much more difficult than a Smith machine squat.
When you engage in a barbellsquat, it ensures your body does so much more than simply push the weight straight up and down along a fixed bar-path.
Also, the fun that comes with the use of free weights is massive.