What's the plate of ginger for?

A little joke that my friend told me yesterday:

At a Dim Sum restaurant, my sister's friend looked puzzled when she saw a plate of ginger being placed beside her cup of tea. She asked "Am I supposed to put the ginger into the tea to form ginger tea??"

O.O'''

For those who are not familiar with eating Dim Sum, you're supposed to mix vinegar with the ginger and use the mixture as a dip for the dumplings.