That's the thing, i don't know him. Working as a fitter and welding pipe you use a file constantly, and I've worn out a few over the years. Fitters are usually super hard on files, they're used to chip slag and dress welds, ream and square pipe cuts and holes, and repair threads on bolts and pipes. We use half round double cut bastard files, which are about the most aggressive ones you can get because it's rough large material removal work. We're using grinders and stuff obviously, but a file can fine tune stuff far easier since it's lighter and taking less off so you are less likely to overshoot. Different types of files are used for finer and finer work, so each one is designed for a certain rate of removal and the finish it leaves.
Once upon a time people commonly worked metal completely by hand, first with chisels and saws, then with files, then scrapers, then lapping. Over time, they came up with what works and rules for doing stuff a certain way, for example only cutting forward. It seems dumb yes, but by cuting forward you are doing your best to protect the cutting edge. When you're in soft material it doesn't matter, but what happens when you hit something harder, say a chunk of welding slag? The slag is like glass, and harder than the file. Drag it backwards and your new file is now trash.
So stay away from welding slag and you're fine right? Not exactly, and chainsaw chain is a great example. It's very high carbon steel, hardened and tempered. What that means on a metallurgical level is some very hard steel is dispersed in a putty of softer steel, which means your file will be hitting those hard parts which are buried in the soft part. Basically like an iceberg, most of the chunk under the surface, but with a huge tensile strength. Used in a forward direction the teeth will skate, backwards they chip. If you're filing soft stuff no problem, but in different materials you'll hit stuff. Another example is cast materials, they have sand in their outer layers, which is way harder than the file. So if you only file foward you're good no matter what you're working on, so that's the way it's taught.
This ability to slide undamaged over harder materials is used when you make a filing jig, aka a poor mans milling machine. Grade 8 bolts are super hard, and so files don't cut them hardly at all, they just glide over the surface. So you use the bolts as you would concrete forms, basically as a screed height for the file. Once it's sliding on both points you're flat across between the 2 bolts. By knowing the threads per inch and number of turns you can predictably move the stops precise distances, helping make sure you don't go too deep. Roller bearings are the next step up on this technique, with even less file wear, but the bolts work fine today as they did 100 years ago.