"It's raining now!"
Yes, it is needed.
This sentence comes with an exclamation mark. So the speaker was probably not expecting the rain, or it was not raining a while ago. To indicate a change of status (not raining becoming raining), 了 is definitely needed.
It is raining now. I want to take an umbrella. (I am only describing the status now. I am not saying about a change.)
It is raining now. I would better take an umbrella. (There is a change of status. It has become different from it was previously, or from my expectation.)
了here implies change, it wasn't raining before but now it changed and it is raining.