My understanding of why injection is used... Two main reasons...
First in commercial production of today's bacon, it just allows them to cure faster by injecting cure in all over across the belly, etc... With they way most of us do it, the reason we wait 5 to 10 days is because it takes that long for the cure to penetrate the meat fully. ( rule of thumb is 1 day per half inch thickness ).
Second... similar to the first, it's in some ways about how long it takes for cure to penetrate the meat... if you have a really thick cut of meat, you can inject into the middle so that some of the cure gets into the thickest part of the meat.
For me, when curing pork loin, or pork belly, I can wait the 5 to 10 days. I don't believe the rule of thumb of 1 day per half inch really directly applies to pork loin, in my experience , cure seems to penetrate lean muscle much faster than it does meat that has layers of fat mixed in... I've observed that most pork loins which are easily 4 inches thick, have cure penetrate to the center in well under 8 days, 5 days is about all that seems needed really.
--- back to dy-cure --- In my quest to figure out if there is anything that defines it, I ran across one or two websites ( google how to make dry-cure bacon and you'll find them ), one of which insists that you don't need Nitrite or Nitrate to be safe... that salt is enough. This issue alone is one that's controversial to me... But I error very much on the side of caution and I personally will very likely always add Nitrite to my bacon. I find it somewhat irresponsible where sites more or less make bold statements that Nitrite is not needed... or give formulations that are clearly using a lot more Nitrite than is really needed. Sometime this winter I'll probably make another attempt at making some dry-cure bacon, even if I never get a definitive explanation of what defines it and/or how to do it safely, but what I won't do , is tell you that it's safe, unless I learn a lot more about it and find some sources that are much more clear about the process , what does, or does not make it safe... ----