What for? (để) làm gì

Để làm gì? at the end of a question asks what something is for: Em học tiếng Việt để làm gì? What are you learning Vietnamese for? It's literally "in order to do what?", and the answer usually starts with để: Dạ, để đi du lịch. For travelling.

In speech để is often dropped: Anh mua nhiều nước vậy làm gì? What did you buy so much water for? Without để, the question can sound less curious and more doubtful, as if there's no good reason: Đi sớm vậy làm gì? What's the point of going so early? Said on its own, Lo làm gì! means "No point worrying!"

Watch the place. Làm gì straight after the person is the plain question "doing what?": Em làm gì vậy? What are you doing? After another verb, it asks "what for?"

Tại sao or sao asks for the reason, and is answered with vì (because). Để làm gì asks for the purpose, and is answered with để (to, so that). English "why" covers both, so think about which one you mean.