There is HUGE difference. Sadly, most people do not know that rule.
"Why" asks for the cause, while "what for" asks for the effect.
Here is an example:
Assume that I am slicing some chicken.
- Why are you slicing some chicken?
- Well, the breast was whole (cause) so I am slicing it (effect).
- What are you slicing some chicken for?
- I am slicing som