[yariv]

Having more megapixels allows you to crop more without losing detail and effectively zoom in on different areas on the photo in post processing. It frees you from needing to perfectly compose your photo when you take it and it makes prime lenses almost as versatile as zooms for getting closer to your subject. It has an important effect on ergonomics because it lets you carry around a lightweight prime lens instead of a big heavy zoom and still be able to achieve similar results, or even better ones if you can take advantage of the prime's higher max aperture.

[ekianjo]

more megapixels is only useful if you have very good opticsa though. otherwise you just get more noise out of your pixels.