Why does she need sleep?
Why does Kusanagi need sleep? Or to wear clothes in a private setting? Or to generally do "human" things? She is an enhanced cybernetic being, while not AI, she can exist without the mundaneness of everyday life.
shareI haven't read the manga and only watched this and the second movie so I don't know if this is ever explained. So this is only my own speculation.
Science can't really explain why humans actually have to sleep. But some studies have shown that if someone doesn't sleep for a long time the mind takes a lot more damage than the body. So Kusanagi's body might not need rest but her brain still is at least part human and needs sleep.
As for doing other human things, they probably make her feel more human and she still wants to be part of human society.
Close, very close.
Sleep scientists are more now understanding the biochemical interactions between the brain and the rest of the body. Since the brain is making about 20% most of the energy consumption, it swaps a lot of nutrients and produces "waste" over the day. The night is used for processing the day events as well as purging waste proteins and byproducts to function properly. Therefor all living beings with a brain need sleep to function properly.
To give an analog in the computer world, it's a scheduled defragmentation of the hard disk drive and getting rid of useless, remaining temporary data that was used and stored in the RAM buffer.
Since Motoko has only the brain left, that makes her human, it's natural that the cyberbody and the brain works as like a real human body to even mimick the cycle of sleep and the brain only does, what was programmed by nature.