It's one of those words we throw in to various sentences and exclamations. I can't honestly think of a reason why.
- I'm going to book a holiday to Ecuador.
- Now I'm not sure that's a good idea. You only speak French, not Spanish.
The "now" in that exchange doesn't mean "At this moment in time".
- I'm in a really bad mood so I'm cancelling our trip to the cinema tonight.
- Oh, come on now. It might really cheer you up.
Again, I can't give a valid explanation for the use of "now" but there it is!
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