Why trim fat at all? A good question, especially for those new to brisket, and it really depends on the cut of brisket you buy. There are three basic brisket cuts: a Flat end, a Point end, or a Whole Brisket (commonly called a whole packer).
Brisket flat: As the name implies, it's the flat somewhat squarish shaped muscle on the bottom of the brisket. It has a layer of fat on top (fat cap), and very little fat marbling. Often times the fat cap is trimmed way down or completely off when you buy it. Because it's a tough cut and has little marbling, this is the part of the brisket that is most difficult to cook and keep moist.
Brisket Point: This is the end section which comes to a thicker point, with lots of fat marbling and collagen, making it much easier to keep moist. It just needs lots of time for the connective tissues and collagen to break down and become tender. This is my favorite part of a brisket. It's great for pulled beef.
Whole Packer: This is simply a whole brisket containing both the flat and the point ends with a HUGE, thick layer of fat in between the two muscles. It's this thick fat layer that usually gets trimmed mostly away.
Why trim any of this fat away? Because it's a lot, and I mean A LOT, of fat, which really doesn't do a whole lot to keep the flat moist. Instead it just cooks off and melts away creating a lot of grease in your cooker. Also beef fat is much harder to digest.
For whole packers, I usually trim most of this layer of fat out, leaving about a 1/4" fat cap on the flat.