One of foil's main functions is providing a watertight barrier to trap moisture inside the food. This can be especially helpful when slow roasting because you're cooking in a hot, dry environment for an extended period of time. Without foil, your roast would have turned out a lot drier and you'd have lost out on tasty juices. Brisket also has a fair amount of connective tissue (containing collagen) that will melt down into gelatin. Given sufficient moisture, the gelatin dissolves and provides a tongue-coating, lip-smacking richness that's hard to replace, but you don't get the same effect if you dry things out too much.
Foil also provides a barrier to direct heat. In the case of a roast, that reduces browning on the outer surface of the meat. Ordinarily I'd argue that this browning adds flavorful compounds produced by Maillard reactions, but the "beefy" taste you really enjoyed might have been the unadulterated flavor of the meat. If that's how you enjoyed it, just go with that!
Now, as to whether you'd get the same effect with pork belly... you might, but it's a different cut, with loads of connective tissue and very tough muscle that would benefit from some added moisture. You'll also wind up with a lot more fat as it renders. I'd expect pork belly to take longer to cook, and a final searing or browning step would be a really welcome addition to get a good crispy skin. Here's a recipe that came up during a cursory search, which uses foil to seal the belly inside an aluminum pan, so the concept is a sound one. I haven't tried it myself, so I can't speak to the relative merits of this method versus other forms of pork belly preparation.