You use it when you're making a comparison.
Say somebody does something silly like losing their car keys. Maybe they have a friend that is always losing their car keys. Let's say his name is John. You could say, "You're as bad as John." meaning that you are as bad about losing your car keys as John is. You would say this after the person being compared mentions something about not being able to find their car keys.
You don't have to always say "You're as bad as..."
You could say any variation of "You're as (adjective) as (noun)." to fit almost any situation.
Ex: You're as clumsy as Susan. You're as intelligent as a snail. You're as sharp as a double-edged sword.
This of course works in many many varieties of situations. Just follow the "formula" above.