"What's the point" is an amazingly good question. For me it breaks down to three main reasons:
You'll gain enjoyment in answering the question. A well-written answer is its own reward, regardless of the upvotes gained.
You'll gain a greater enjoyment in the HP canon by exploring it in more detail (note that most of the questions asked do actually have reasonable answers).
It will allow you to take part in a community based around answering questions about HP.
Although it's not the done thing, I'll turn your question around and ask "Why are looking at the Harry Potter questions if you don't think they're worthwhile?"Why are you looking at the Harry Potter questions if you don't think they're worthwhile?"