Took off a few years ago in the 757 on a dark night heading out over the water and flew straight into a rather bumpy CB.
Had not seen any lightning prior to departure, (hadn't seen anything for that matter)
After turning on the Radar we could see cells everywhere and dodged the rest.
I turn it on and leave it on for the entire flight now, regardless of conditions, day or night, you never know when there might be something out there that you would have thought unlikely.