Back to the actual question…
Strafing is typically used when you outnumber on offensive pips the defensive pips, and can therefore attack and retreat causing more harm than harm received. It is especially useful for Russia, both on the german front and japanese front, though I find myself using it against the japanese more often. The best example I can think of is when Japan realizes they must hurry to help out germany. Their front lines are then often comprised of lots of armor, and some infantry. Of course, they have a nice chunk of supplies coming from japan to back them up, so if you take out the front line, whatever you attacked with will in turn get taken out. Having a lot of fighters makes things easy, because you can always retreat when things go south, and even if you take it you only lose infantry, which isn’t too bad. But russia usually doesn’t have 6 fighters lying around. Lets take a very basic example of a naive (or hurried, more likely) japan marching towards moscow.
Their front line is 1 infantry, 3 armor, their second line is 4 infantry, 3 armor, and third line is 3 infantry 3 armor. At some time you will learn when to build offense for russia, but lets say that russia has managed to develop a few tanks (or) artilleries and is sitting in novo. Japan is in sinkiang with their inf and armor, and russia has a stack of 4 inf, 4 armor in novo. So, you can either take sinkiang and get slaughtered on the counter, or more optimally, you strafe. I’ll go directly on pips (which is not realistic at all!) and attack the 1 inf, 3 armor with 4 inf and 4 armor. So, russia can expect to hit about 3 times on the first round, and japan will hit about twice. The tradeoff is 2 russian infantry for 1 japanese infantry and 2 armor. So out of this battle russia has gained 7 IPCs. If you can strafe effectively enough and the other allies are doing something besides picking daisies, russia can sometimes hold off japan for about forever. Compare this to if you try and take his territory in sinkiang. You get countered and everything is destroyed, and you are at an IPC loss against japan’s front. Okay, so lets take the above example. Russia takes sinkiang down to 1 armor, and retreats with 2 inf and 2 armor. In moscow they build 3 armor and from the previous turn move in three more inf to novo. Now if japan moves all to sinkiang you have 4 inf, 4 armor vs. 5 inf, 5 armor. So now if russia strafes again it is 20 pips vs. 20. Lets say each gets three hits each round, after the first it is 1 inf, 3 armor vs. 2 inf, 5 armor. Russia stays on and destroys 3 more, and japan gets 2 more, russia retreats with 5 armor (then moves more inf from mos->novo). The key is that russia is trading their infantry (and less of them), for japanese infantry and armor with russian inf. The IPC differential of this battle puts Russia up 12 for that battle alone.
Strafing can be brutal though, and it’s sort of personal preference and how lucky someone is feeling how they strafe. You don’t want to overstrafe because then you lose a lot on the counter, and if you understrafe it is pointless. The general principle is still that you want to produce the capability for you to trade less of your ipcs for more of your opponents.