Well, I can only speak for myself, but the 640ND checks a lot of points for a carry snub. Compact, like all 1-7/8" J-frames. All steel, so more durable than the lightweights with less felt recoil, but still light and compact enough to be easily carried, especially on the belt. Enclosed hammer so less opportunity for crud to get into the action. A little lighter and slightly more compact than the J-magnum-framed 640-1, thus a little more size-efficient for someone who only intends to use .38 Special ammo. No lock. While I don't hate on MIM parts like some purists do, I do like that it's a non-MIM gun.

And, for a less rational reason, Ed Lovette, a former CIA paramilitary operations officer who wrote the classic book, The Snubby Revolver, which I'm a big fan of, has said that the 640ND was one of the sidearm options for those CIA operatives authorized to be armed. To me, it adds appeal in addition to all the practical reasons for wanting a 640ND.

Though I will add, a DAO 649 in .38 Special would be a close 2nd.