In the following sentence:
Не нужно объяснять, чем так хороша физическая активность зимой.
I didn't understand what this чем meant, so I tried using Google Translate to get the feel of the structure.
No need to explain why physical activity in winter is so good.
It seems to mean an interrogative pronoun, but in my dictionary, there is no such usage. It only lists чем for comparison.
Another use of чем is as an instrumental case, but объяснять seems to take an object as an accusative, so it should be что, not чем, I think.
So I think I'm missing something. How does чем work here? Can I use it like почему?