It’s dead simple to explain. (I hope. If not ask specifics.)
The short version is that it’s a polymer bullet jacket instead of a lubed lead bullet or a lead bullet with a copper jacket.
Lead by itself will lead the bore; that’s a huge problem. 300 years ago the lead ball was kept away from the steel bore with fabric and grease. The faster one pushes a lead bullet the more the problem of leading comes to the foreground (I’m leaving out a great deal). Jumping to today(ish), harden the lead by adding tin and leading is reduced and velocity is upped. But tin is expensive. Or one can lubricate the bullet by adding a lube to the lube grooves of a bullet to reduce/prevent leading. One can also make bullets with a pure lead core and a copper jacket with the right dies and press (it’s called swaging), but that can get quite costly. Rather than swaging lead into a jacket one can cast a lead bullet, coat it in a polymer, and that will, after sizing it, result in a “jacketed” bullet that won’t lead the bore up to substantial velocity with minimal cost and little $ outlay. Backing up, old school lubed bullets work, I’ve fired thousands of them, but they are dirty and produce smoke. Coated bullets are clean and don’t smoke. Another benefit is that if one isn’t pushing velocity one can pretty much ignore lead hardness as it refers to leading since the coating contacts the bore and not the lead. Think of it as a bullet jacket. If one uses a sound suppressor, the can stays clean.
When I was researching how to produce my bullets I found all sorts of reasons on the plus side and very few on the minus side. One is start up cost.
My gear consists of the small bottles with nozzles to hold the mix, a discarded HDPE freezer container to mix the bullets and mix, a few ice cube trays, and my oven is an old toaster broiler that we replaced. Also some 1/4" hardware cloth to allow the coated bullets to dry on before going into their cubicles in the ice cube trays before being baked. The ice cube trays are me gilding the lily to keep the bullets separate (not needed but inexpensive).
All of the advantages of a jacketed bullet but done at home with no sophisticated gear. One gent told me that he was pushing them to 2500 fps and getting sub MOA @ 1000 yards (or was it meters? IDR).
The bottom line is that coated bullets do much of what jacketed bullets can do with a much less costly cast bullet made from wheel weights and done at home. Would I push them to 4k fps? No.
If purchasing commercial bullets, coated bullets save A LOT of $. I’ve used them for years I just never thought to make them myself until antifa and biteme produced the bullet shortage of a year ago. When I write “bullet” I mean the cartridge component and not the hellyweird bastardized version of the word meaning cartridge.
Just a word of warning… if you think that you’ll ever need to make your own bullets and live in a state that still uses clip on lead wheel weights buy all you can find now. I bought mine 15 years ago (1500-1800#) and due to my age it’s a lifetime supply. I never thought I’d ever need them but bought them “just in case”. The “Cast Boolit” forum has an excellent thread that shows what is usable and what is junk. Tip: zinc will poison your lead, but it can be removed.