A meatcutter in a large packing plant can fabricate an enormous amount of meat in a single day. They have the advantage of the "assembly line" arrangement with all of modern techniques and equipment. For instance, the task of boning a leg of lamb and tying it into a roast is done over and over by a single person working off of a moving conveyor belt. The cutter doesn't need to carry anything or move from his or her spot. The its a simple 1-2-3 step proceedure, pull the leg off the conveyor belt, bone it out and throw it back on, and then the roast is formed and netted by someone else. On larger cuts, such as beef, the task may be aided by hydrolic pullers and tools specifically designed to reduce stressful labor and increase speed of fabrication. The result is a product that is cut accurately to industry specs that is standardized and available from purveyors with regularity. The same goes for portion cut items. Steaks and chops are cut on machines that can regulate thickness and weight and done in huge volume so the processor can pick out all of the like-sized portions for accuracy. You can buy a perfect 10 oz striploin steak over and over.
So why would a chef decide to do any fabrication in house? There is no way to match the speed of the large plants and they would need to pay someone to do it. But the restaurant chef that has some butchery skills has an advantage over those that don't. They can custom cut things that are outside the norm such as instead of simply boning the leg of lamb they can french off the end of the shank, remove all other bones, stuff the interior and then tie it to create a look that is unique. Meat cutting, especially on the small custom restaurant or meat market level can go in directions that a large processor can not possibly do. Other examples are chefs who dabble in curing meats. A ham, custom partially boned in the true prosciutto style, leaves the femur and shank intact but the aitch bone removed except for a little corner of it to protect the interior from mold. You can't buy this from a large processor. All sorts of little tricks and custom ideas can be done to maximize yields, create unique looks, convert tougher cuts to tender all by butchery skill.
Another situation invloves chefs that want to sell meat from a small local processor. Most small processors cut meat from local farms but cut for retail freezer orders. They focus on quick volume band saw work which may not be good for a restaurant. Today we find some chefs buying the entire primal cut and breaking it down themselves. This enables them to utilize all of the trim, bones and even fat to make a large variety of sauces and dishes that wouldn't occur if they purchased pre-cut items. This cutting requires a lot of skill and time. There is no assembly line. Meats need to be cut on a large table instead of off the hook. Meats are out in a warmer environment than in a meat plant so spoilage is an issue. Crosscontamination can occur if proper care isn't taken. But the end result can be a taste and texture that is as signature as a fine pastry or a great sauce. A restaurant that custom cuts their meat can use the fact on the menu or in advertising. Even an inexpensive bistro can offer an in-house ground burger that is made from whole muscle cuts rather than buying pre-ground beef that may have contamination.
I must admit that I am partial to chefs cutting some or all of their meat in-house, afterall I have trained a lot of them. I also realize in many situations such as very high volume hotels or casinos can't possibly do all their fabrications but there are often ways to save money or create new presentations by custom cutting.