Because a direct word-for-word translation of

"Now winter?" or "Now September?" is grammatically incorrect English.

Conversely, the present tense verb "is," in Ukrainian is omitted, so you're left with one word sentences. "Зима?" "Вересень?" This sounds incomplete in Ukrainian, so the word зараз, meaning "now," is added for clarity. Another alternative could be Це зима? Це вересень? More literally "is it...?"/"is this...?"