In a couple videos last month about American .22 LR rimfire training rifles, I got a surprising number of comments from viewers who did not understand why a military would train with a .22 caliber rifle instead of their actual issue cartridge. There were enough of these comments that I decided it would be worthwhile to make this short video explaining the rationale, because the question suggests a fundamental misunderstanding that should be addressed.
Basically, it is done because learning to shoot with a small and very light cartridge allows the trainee to understand and practice the fundamentals of shooting (sight picture, sight alignment, trigger control, and followthrough) without the distraction of recoil. What seems to be fairly widely misunderstood is that those fundamental skills translate extremely well between different guns and different types of shooting. Someone who is a good shot with a .22 will have a pretty easy time adapting to shooting a more powerful rifle like a 1903 Springfield, while a novice shooter introduced to the Springfield as their first rifle will have a much more difficult time learning proper shooting fundamentals.
This, of course, applies equally to civilian shooting as it does to military training.