Interesting question. To make an answer as brief as it is possible: in Polish adverbs do not only describe activities, but also answer the question "how is it like?". As long as u describe sth impersonally/indirectly (situation, weather, etc.) in Polish, u use adverbs.
Byłoby wspaniale. ALE One były wspaniałe.
Tu jest dobrze. ALE Sytuacja jest dobra.
Tu jest gorąco. ALE Atmosfera jest dzisiaj gorąca.
Let satisfy ourselves with this short explanation and do not go meticulously into the philosophy of Polish language. I`m sure that u`ll get it easily during ur studies of Polish.