The reasons are many, and aesthetic is a small but important one: Just like serving a bone-in roast or a whole chicken—as opposed to meat or poultry cut up into parts—a heftier cut of salmon presents more impressively every time and also seems more special occasion-worthy for dinner parties. More importantly, salmon cooked in larger, family-style pieces benefits the cook in ease and the eater in flavor and texture. Using a broader piece protects the interior of the fish, keeping it from drying out in the oven, or on the stovetop or grill, and the hands-off method of cooking I've outlined below means less chance of broken or torn skin come serving time. Plus, buying from places that sell bigger cuts—rather than precut, packaged, or frozen fillets—means you're more likely to be purchasing fish that's fresh, and from a source with a lot of turnover. The fishmongers that have these larger sizes available tend to be the ones that break down the whole fish themselves, meaning you're starting out with better-tasting product overall. In other words, if you buy in bulkier pieces, you'd be instantly better at cooking fish, and probably cook a lot more of it.