Generally, I brine for moisture and inject for flavor. There is some overlap though as injecting will indeed infuse moisture, but in a mechanical way. Brining is more of a chemical process.
Protein cells absorb water, salt and flavor in a brine. These cells then slowly release moisture during a cook by dehydration. Injecting forces moisture between tissue and muscle layers, not necessarily into the cell structure of the protein. Sulfates can be added to either increase the holding capacity of each protein cell.
Indeed brining and injecting are two different techniques that don't entirely achieve the same result, even as it appears that they might.
Traditional Santa Maria Pit
Mini, Small, Medium & Large BGE
Weber 22" OTS & 26" OTG & 18"WSM & WGA
Each wearing custom handles from Marty Leach