"How hot are summers here?"
I thought of it as summer has how much heat? Without the 有(have) it's summer how much heat which seems a bit incomplete. This is my speculation :)
Without the 有 it id the statement "The summer here is very hot." I don't think it can be a question without the 有.
BTW, we consider this is a question is because "多" this word, not "有".
多熱？ How hot?
多久？ How long? (timely)
多少？ How much?
It also can be a question without "有". In my opinion, I think "有" in this sentence can emphasize how hot it is. Before you ask the question, you might know it's hot here, but you wanna make sure how hot it is.
這裡夏天多熱？ 這裡夏天有多熱？ are both correct in Taiwan. I'm not sure in China. Anyway, I don't think it would be a problem for native speaker.