Because everyone's scared of the dark at some point in their life.
I don't own YGO, or even a nightlight but my sister owns a lava lamp. want it?
The soft glow of the canary shaped nightlight cast a shadow on most of the room, lighting up only the bed it was plugged in next to.
"I don't see why you need this thing."
"I'm afraid of the dark."
"Five year olds are afraid of the dark. Your fear makes no sense."
"Ir's called a phobia, Yami. The literal definition is that it doesn't make sense."
Mutter mutter. "Well, whatever. I still don't get it though. I'm dark. My name means "dark":"
"Yami, just please go to sleep. I have school in the morning."
"Fine. But I'm turning it off when you're asleep."
"You can't. I'm afraid of the dark."
"You're sixteen! And you'll be asleep!"
"I'm still afraid of the dark!"
And Yami Yugi just muttered quietly to himself as he watched Yugi climb into his bed and close his eyes, smiling.
And as he turned to go to bed himself, he passed the canary shaped nightlight.
He could have sworn it winked at him.