Other than for the hobby/fun of it; and specific MATCH SHOOTING (Which most match shooters reload no matter WHAT gun/caliber they have); there really is no other reason to reload for the M1 Garand.
There is plenty of plinking ammo available. $96 for 192 rounds from the CMP. Basically, $10 a box with S/H. If you really want to hunt with the rifle; it weighs a ton; there are 2 options. Spend $35 and swap out the gas plug with a schuster plug, and then you can shoot ALL the Commercial Ammo you want to. Or, spend $5 at a Army/Navy store or gun show for a 2nd gas plug. Drill out a big-ass hole in it. Swap that out for the original plug. Then, you've got essentially a bolt action gun. Shoot any ammo you want, and don't worry about adjusting the schuster plug for each different type of commercial ammo. In a hunting situation, you MIGHT get 2 shots at an animal. If you're on shot #3, you either suck at hunting, or shouldn't have been taking that 1st shot. I hunt with bolt actions 99% of the time. (As does most hunters). Manually chambering a round in an M1 Garand, is no different than chambering a bolt action rifle. At least for me.
So unless you feel compelled to hunt semi-auto; or you are into match-shooting; there really isn't any reason to reload except for the hobby/fun of it. And if you're already reloading for another caliber, then it can also save you money. If you are starting reloading as a new hobby, there will be about a $400 initial investment. But that pays off relatively quickly.