I do not inject butts have tried and didn't notice much difference if any.
I do inject briskets and find they come out with more juice in them don't think it adds much to the flavor to them but maybe a little
There is a competition technique called a 'late injection', and you do this during the stall or before you wrap the big meats, or at the end of a chicken cook. You don't use your early injection liquid because that is too strong and likely heavier on the salt.... which has time to cook down during the first 5 or 6 hours the meat is on the pit. The late injection is just a flavor bump and helps with moistness. Think of it as a savory beef or pork au-jus that compliments the meat. A late injection on chicken thighs can be something as simple as a butter broth. I've judged chicken that was so juicy it squirted when taking a bite.
I thought the meat might not lose so much of the water and it might be safer if the internal temperature is above 140 but mostly the water effect. Anyway it sounds like this is done in competition sometimes.