Running gates will make you a better skier because the race course forces you to make exact turns. The more sloppy your technique, the more difficult it will be to make those turns with any precision.
The recreational pay courses of Nastar are perfect for practicing your turns. They're usually on mild blue runs, the sets are always a relatively easy back-and-forth giant slalom, and Nastar gates can be run without much concentration--so you can fully concentrate on how you are skiing, and what you need to do to turn better or go faster.
This is especially true if you are a club or Masters racer. For example, the newest technique among World Cup racers is to hit GS gates like they are slalom gates. It's a more direct and faster line. But a lot of recreational racers are wary of getting that close to double-paneled giant slalom gates, or bashing them, when they're not designed to give like slalom gates. Nastar is great for learning how to do it.
You can run the same course over and over again, getting closer to the gate with each run. You'll learn the feel of hitting the gate without bruising your arm or shoulder (though some sort of padding is a good thing). You'll learn how to angulate over to hit the gate with your arm without hooking a ski. By practicing for several days, you'll get the technique wired in.
Even if you're not a racer, repeatedly running a simple course on a daily basis will make you a better skier. On a regular run, you're making turns all over the hill. In a Nastar course, you're making the exact same turns every time. You get used to them, allowing you to push your envelope.
Maybe you've never been able to bend your ankle over enough to set your ski on edge for a powerful carve. After learning how the turn feels on the third gate or the fifth gate, you can try edging more, then a little more, and suddenly you're carving and even accelerating out of the turn.
The timing clock is an accurate reporter of your progress. Naturally, you'll take a second or more off your time once you've run the course a few times. But once the clock doesn't change by more than a few hundredths of a second, that's when you should start working on your technique.
Are you significantly faster if you deliberately hold your inside arm higher as you pass the gate? Are you faster if you buckle your boots tighter--or looser? What does the clock tell you when you take a run looking further down the course, or skating through the first few gates?
The more precise your technique, the faster you can go. Some Nastar racers can go fast, but without much control. Before the finish, they have to jam their edges to stay in the course, scrubbing speed. The buddy they race on a dual course kicks their butt.
It's better to practice going slower under control, gradually working up to faster speeds. Use your physical senses; you can easily tell when a turn feels good or bad, you can hear the noise of a skidded turn and you can feel the bouncing that happens when you're leaning back instead of forward.
If you're able to do ten runs on any one day, you'll improve more than you would with a private ski instructor lesson. If you're not already an expert skier, read up on ski technique so you'll know what to work on.
There are many online sites where you can find excellent information on ski technique and training, but nothing beats real hands-on--or rather, 'feet-on' experience. And for that kind of experience, nothing beats a Nastar course.