干什么用英文怎么说?

What is used for in Engpsh is referred to as "What is its purpose/function/use?" in Engpsh.

This phrase comes from the common way to ask about the intended or specific role of an object, activity, or situation.

Here are some examples in Engpsh with Chinese translation:

What is this pencil used for? (这支铅笔用来干什么？)
What is the use of having a meeting if no decisions are made? (开会没有做出决策有什么用？)
Can you tell me what this tool is used for? (你能告诉我这个工具用来干什么吗？)