The main draw of a smith machine is providing stabilisation for your weights. They typically include a barbell with guide rods, so you don’t have to put any energy into keeping the weights in the right place. This allows you to lift heavier, as you’ll be able to put all your effort and strength solely into lifting.
They’re also great for safety because they mitigate the need for a spotter. This is ideal for those who want to lift barbells at home, where a spotter might not be available. Most smith machines allow you to hook the barbell onto the frame if you find you can’t complete a set.
Finally, they help build good technique, because they lock you into the right position, promoting better form and teaching you how a proper lift should feel when completed with free weights.