Why do the Olympic racers hit the flags along the course in a downhill ski race against time? Doesn’t this slow them down? Or, does it hold them on course, perhaps? (I notice not all flags get hit, and some skiiers hit no flags at all).
What’s the SD?
The don’t usually on downhill racing, they do all the time on the slalom. The slalom consists of many tight turns in which the skis must go through gates (marked by a pair of flags on poles). The body itself not actually go through the gate. Often the skier is turning right then left immediately. The fastest way to do this is to move the body as little as possible and get the skis to just cut through the gates. The body then often hits the poles. You’ll note that they are flexible so they bend out of the way when struck and pop right back up again.
Because to get the best time, they want to ski the shortest distance. That tightest line means the closer they come to the gates, the tighter the apex of the turn, and the shorter the time. Going even a little wider away from the gate will lose them a hundreth of a second or so, and every hundreth counts.
Coming that close is a gamble, becuase if they go on the other side, or even straddle it, they are disqualified.
At those speeds, it really does smart to hit 'em, but them’s the breaks. If they weren’t tough and nuts, they would not be racers. 
In other words, if you ski far away from the gate, you make a rounder turn. It has a shorter radius. That means you’re putting more of your speed going across the hill rather than down it. The opposition of your ski edge to the hill also slows you down. So, not only do you travel further, you travel slower.
Another way to think of it is that coming closer to the gate allows you to ski in more of a straight line, which of course is the shortest distance between two points!
I race recreationally, and my experience is that hitting a gate in GS is hardly noticeable. I have seen racers singe their speed suits or parkas simply from the friction!
On the other hand, the extreme impact that goes on in slalom hurts like a mo-fo, unless you wear armor. I wear a padded “shirt”, shinguards, padded gloves, pole guards, and a helmet with a mouth guard. I practice “blocking” the gate, which is hitting it with my arm/hand/pole so I can ski my body “over” it. The idea is to carve through the turn, with the inside edge of your inside ski right next to the gate. If you’re carving correctly, your shins should be coming over the gate itself and the rest of your body should be on the other side of the gate. That’s why you need to block the gate, so your body can go over it.
Of course, that’s the theory. I doubt I’ll ever get there. As a good friend says, “I do a lot of things you shouldn’t do.”
Like others said, you want to keep your center of mass moving as directly down the fall line as possible. Often this invovles hitting the gates either with a shin & cross block like in slalom, or with a shoulder in the other events.
Here is a good example of a slalom racer blocking a gate. You can see that pretty much just his skis and boots are actually on the inside of the gate.
In GS, it’s typically fastest to be shouldering the gates. That’s why speed suits have arm-pads. Even so, when I was racing in college, my triceps were permanently bruised from GS training. In the speed events, the fastest line isn’t necessarily to be right on all the gates. It’s tough the tell on TV, but if you pay attention you’ll usually be able to pick out some gates that most of the racers are right on and some gates that they’ll take a wider line on. This varies pretty drastically from course to course. In the Olympic Sestriere course, for the most part, racers took a fairly round line to carry more speed and/or set up for jumps. On the other hand at the World Championships in Beaver Creek in 1999, Hermann Maier practically shinned every gate in the Super G and won pretty handily.