As others have already said, adding 了 indicates a change of state. Here is an example of when you might want to add 了, and when its not necessary:
If you don't have a watch and are wanting to ask a friend what time it is, you can simply ask 现在几点？
Now imagine you are waiting for midnight on New Years eve. You ask your friend what time is it? 现在几点？Then 10 seconds later you ask again, but this time you ask 现在几点了? You can add 了 this time, as we know the time has changed but still don't know exactly what time it is. You can imagine it in English, "What time is it?" (10 seconds later) "What time is it now?" or "what about now?"