"Now" is just as necessary as "сейчас". If the Russian sentence uses the word, then it's best that the English one does too.
Anyway, it does change the sentence slightly, mostly the emphasis. Say the two of you are leaving for a dinner party later that day. You might ask, "are you shaving?", which would imply "are you shaving before we leave?", or you could say, "are you shaving now?", which would imply more like "are you shaving now instead of right before we leave?".
Basically, it emphasises the "now".