So why would airsoft beat out pellets and metal bbs for those things?
Two really good answers above, but I'll add some.
1, you can do actual force on force training safely with airsoft. Even a low velocity pellet rifle/pistol is well into the "unsafe" category no matter how stupid we were with the "two pump limit" with BB gun wars as kids...
2, Airsoft replicas are often "functionally identical" or "pretty darn close" in terms of magazines and semi-auto fire. BB/Pellet guns even if visually the same, generally don't have magazine similarities.
3, Pellet rifle and pistol triggers are generally "really good" and much lighter and crisper than centerfire or rimfire firearms, where Airsoft triggers do a better job replicating the feel of a firearm trigger. This is a generality, because I haven't handled every firearm and air rifle/pistol out there, so I'm positive that there are exceptions to this.
I hope this is helpful, and I still recommend air rifles and pistols to people who want to be more accurate shooters.
Jimro