Every tool has its place, and weights are nothing more than that – tools. The tool you use and how you use it depends on your goals. Generally, free weights are more useful for developing strength, and isolation machines are more useful for either rehabilitation work or hypertrophy.
Depending upon your workout program, there are outliers that can cross over between these general rules. Again, it all comes down to your goals.
Free weights demand a higher neural recruitment rate, challenge your balance, and follow more natural movement paths.
More bang for your buck. Free weights will provide you with more stress per exercise than any isolation exercise. Look at a barbell squat, for instance: you are bracing your entire inner unit musculature, your spinal erectors, your lats, teres major and minor, as well as activating your glutes, hamstrings, and quads to perform the actual movement itself.
The closest approximation to a barbell squat you can do with a machine is a leg press. With this you are still focusing on your hamstrings, glutes, and quadriceps, but without any of the other stabilizing muscles that must be active to keep good (safe) form.
The result is that the leg press (machine weight) stimulates far less muscle activation and produces much less stress on the body. It is that exact stress that you need to develop functional strength and/or to build more muscle. Note that the volume of your workload when lifting will directly affect whether you build strength or size, as the two are not necessarily directly correlated to one another.
Machines, though, are generally safer and so long as they are used properly, they ensure proper technique.
Safety is a huge point to weigh in on, here. Let’s look back at our barbell squat vs. leg press example. If you have an arm in a sling you can’t very well perform a barbell squat. You can, though, still leg press.
Similar, with an injured back you might not be able to do standing bicep curls, but on a seated isolation machine you don’t have to worry about aggravating and/or exacerbating the injury.
Let’s say you just recovered from a pulled tricep. You could go and try to benchpress to build up your arm to where it was, but this would be extremely inefficient. All of the muscles involved in that exercise can be worked more directly for better results. Rather, it would be more beneficial to isolate the tricep with resistance appropriate to that muscle. A machine that isolates the tricep would always prevent you from cheating on your form to compensate with any other muscles to help the movement along.
Keeping with the advantage of isolating muscles, this is a key point for bodybuilders or those looking to build up their body’s in a specific way. Whether for fun or competition, this is no different from creating a sculpture, and adding clay to the figure to balance out proportions. The only difference is that your own body is the sculpture, and the clay being added is muscle mass.
The ability to completely isolate which muscles are being stressed allows you to target specific points on your body, your sculpture, to which you want to add more muscle, or clay.