Why do chefs use Gold and Silver Leaf?
It's just decorative, in a bit of an extravagant way. The leaf is so thin that you can't really tell it's there when you eat it; you're not really eating much metal. That's also why it's not that expensive - for example I see 25 sheets for $49 on Amazon. Not cheap, but if someone uses one $2 sheet for a few dozen truffles sold for a dollar each, it doesn't seem quite as crazy anymore.

It's also not actually that new an idea. For example, kaju barfi, a traditional Indian dessert, is commonly decorated with silver leaf. See Google image search for pictures - it does actually look pretty cool.