buzzy: to keep her from sliding into the ocean and drowning, I'd guess.
Either that, or as revealed in the later parts of season 2, at least some of her state (her blindness at least) is psychosomatic, a mental reaction to witnessing the murder of her mother.
Anonymous3: actually, she never witnesses the murder itself, V.V. had her mother killed then put Nunnally underneath her body to make it look like a terrorist attack, and Charles cast a Geass on her to make her lose her sight. I don't know about why she can't lose her legs, but that adds to her charm. she's still sexy with or without her sight :P
Anonymous4: She was probably crippled to begin with because even after the Geass her dad used on her was broken (which I honestly think was half because of her willpower and because the person who used it was dead which may have made it easier) but I'm pretty sure she was just crippled from the start
Either that, or as revealed in the later parts of season 2, at least some of her state (her blindness at least) is psychosomatic, a mental reaction to witnessing the murder of her mother.