Lots of reasons - trying to minimise propellant/gas noise whilst increasing performance over a 22LR. Super useful if you don't have the funds to buy a 17 HMR but have a 223. Not sure about the OP's motives, but I'm trying a similar thing as the properties I hunt on aren't large. And if I can reload for similar money to 17hmr ammo then that's an extra bonus as I like to reload and also experiment. More time shooting one rifle is good.
I've also found reduced loads useful to get beginners comfortable with shooting a bigger rifle than they're used to before introducing the full power loads. I know in the case of the 223 this is minor, but for someone totally new to firearms it's good to work up regardless, particularly if the rifle isn't suppressed.
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