Pardon me while I ping my wife to this thread... she may be able to provide a Texas barbecue perspective (she's the cook in our household.)
However, if I had to guess ... I would expect that, being "bone meat," ribs cook quickly on the grill, and probably dump all their fat. Pork on the grill has a marked tendency to dry out before you can cook it through. Parbroiling --- that is, cooking for a few minutes in just enough water to cover --- would help cook the meat and retain the fat and moisture. Less time on the grill means more fat and flavor in the food and shorter cooking time. Plus, you can impart extra flavor to the meat during the parbroil process (garlic, etc.)
As my husband will tell you, I have a great fondness for garlic ...