Peas change the taste of a tuna casserole by adding a sweet, fresh burst of flavor to the dish. Including peas in a casserole gives the dish a healthier taste.
Peas also serve to brighten up the look of a tuna casserole, as the dish tends to be a bland color if there are no added vegetables. Most tuna casseroles call for peas, but they are not an essential ingredient. Peas can simply be left out or substituted for green beans, bell peppers, carrots or corn. This dish became very popular in the 1950s and is an affordable, one-pot meal.