To start with - there's:
FOILING DURING COOKING
Little to no smoke will get at your meat while doing this, so you may as well toss it in the oven. However, for some meats (like well-trimmed brisket flat), letting the meat spend some cooking time in foil prevents the juices from cooking-off. Instead, the meat "steams" within its own juices. If the meat you're cooking has high fat content, and if you're doing a low-and-slow cook, you probably do not need to do this.
POST-COOK FOILING:
A lot of the stuff you will cook on a WSM may benefit from this for several reasons.
Time - Timing of barbeque can be difficult. You can "ballpark" it with time and experience, but generally, "it's done when it's done". In this case, for large chunks-o-meat, I recommend planning so that the meat can be done up to two hours before you need to serve it. If you wrap your brisket or butts in foil and place them in a cooler, lined with old blankets, they will stay nice and hot for several hours - allowing you to pull or slice and serve when needed.
Juiciness / Evenness - The post-cook foiling, even if only for about half an hour, gives the meat a chance to re-distribute the juices within.
So far, I like post-cook foiling for butts and brisket flats.