Q: I know free weights are the best tools for building muscle, but are machines ever useful?
A: Free weights should always be considered your weapon of choice, but machines have their place—particularly if you're training purely for aesthetic results.
Your workouts should begin with compound free-weight lifts, but as you fatigue, you can finish off with machine movements that better isolate the target muscles and allow you to pump them full of nutrient-rich blood. For example, your chest workout could start off with bench presses and dips and then end with a few sets of pec-deck flys or machine presses.
For the greatest gains in strength, you'll need to rely on heavy, free-weight movements almost entirely, as these allow your stabilizer muscles to work to full effect (critical for lifting big loads). But if all you're interested in is big muscles, you need to drive as much blood into the muscle as possible, and machines can allow you to do this safely and with good form.
Machines are also a key back exercise. If you have trouble feeling your lats on rows and pulldowns, or you simply aren't strong enough to do pullups and chinups, lat pulldowns are a perfect substitute.