day25_What purpose does it serve

一.句子：What purpose does it serve?
二.含义：它是用来干嘛的?(功能、作用)
三.对话：
A: Why are you taking vitamin C all day long?
B: They are really good when you're sick.
A: What purpose do they serve?
B: They help strength your immune system.
四.扩展：
1.表达某个物品的功能是什么，还可以使用:
what is it used for？
what is its function?

Thank Coach Shane