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I think it's basically a personal preference. Some choose to inject others chose to brine. Injecting is done for any number of reasons-mainly though for flavoring.
For Curing purposes, you should inject curing brine into meats greater than 2" thick so it is curing from the inside-out and the outside in; otherwise you risk 1) the curing never reaching the center of the product, and 2) the uncured innermost part of the meat spoiling before it can be cured.