If a narc has someone else, why does he/she still bother an ex?

This question has nothing to do with me, it's just something I wonder about when I read various stories on here, whereby the N has left the person for someone new, yet still hoovers the former lover/spouse.

Unless there's kids, there's no need for contact.

To the best of my knowledge, my ex N wasn't cheating. But, I say 'to the best of my knowledge.' lol

I left him, when the abuse got to be too much...and so I never really dealt with my ex leaving me and then also coming back to me, while he had someone new. I started dating shortly after the break up (way too soon) and then I had heard about a girl he started dating after that. He really devestated her from the sounds of it. This was all last year, and I have since not been in contact with that whole social group.

I read a lot of stories on here, where the ex broke up with some of you for a new person. But, they are still coming back around to you...telling you he/she loves you...or saying nasty things...hoovering in general.

Why do they hoover if they have 'secured' new supply?

I have read about this disorder quite a bit, but this part never touched in my personal experience, so I guess when I read these stories, that part of it makes little sense.

Is it because they just wish to have endless attention? Triangulation?