What is a 50mm good for?

Yesterday Alex asked me what is a 50mm lens good for?

Here are my feeble attempts to answer him:
1) 50mm x 1.6fov = 80mm ~ short telephoto ~ half body portraits
2) f/1.8 ~ large aperture ~ low light no flash photography
3) f/1.8 ~ large aperture ~ blur background

Any more justifications?

4) It's dirt cheap! Go get one now!