During the post-war boom, Christian Dior said: "A little black dress is an essential element of a woman's wardrobe." 'Nuff said.
Why do LBDs look so good? Because: they're sexy, black flatters your figure, they're easy to accessorize, they never go out of fashion, they go with any style, they're difficult to get wrong and just putting one on somehow makes you feel like a goddess.
Every woman needs at least one LBD (if not seven!) for day and night, and they're an excellent investment because you know you're going to get a lot of use out of them.