I compare musical composition to architecture. The key, meter, chords, and scales are your structural materials. The arrangement is your facade, the melody your utility, and the lyrics your furnishings. A great song is structurally sound; it doesn't have to be complex, but it does have to be aesthetically pleasing. A construction of very poor substance, even when covered by a fancy finish, will soon reveal its poor workmanship.
Furthermore, I compare musical arrangement to cooking. The melody and harmony are your recipe. The instruments are your ingredients and the modes and beats are your seasonings. Just as how certain foods and spices are inextricably linked to specific cuisines, so too are certain instruments indicative of their style. (Example: adding the Sitar to "Norwegian Wood" was akin to adding Curry powder to an otherwise run-of-the-mill meat and rice dish.) As the skill of the musician or the quality of the food increases, so does the excellence of the finished product.
