The reason bananas are curved has to do with how they grow.  Bananas grow in bunches known as "hands."  As the bananas in a hand mature and grow larger, they eventually curve upwards so that instead of pointing towards the ground, they bend up and point towards the sky!  The ability to do this is pretty neat, and it's known by the complicated-sounding term "negative geotropism."  Most likely, the reason that bananas have evolved to have negative geotropism is that it allows them to reach out towards the sun and get more light!