Machines only work specific muscles and dont get your whole body involved. This is important not only in working different areas, but also for getting (i believe anyways, someone correct me if wrong) ATP circulating in the body and building muscle. Its more complicated than just "this works this muscle." The range of motion on machines are set and offer no room for natural movements and, as above said, they do not build stabilizers at all. If you have injuries, are really really old and fragile, or unwilling to learn then I guess machines are cool.
Just google "free weights vs machines" lol.
I'll personally say that I worked out on machines for a couple months and the results were weak. Once I started with a routine with free weights I got awesome gains. No machine can replace the squat or deadlift.
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