Especially if you don't have a model/aren't comfortable photographing yourself wearing the necklace, it's a really big help. Even the most lovely photos of a necklace laid flat won't help the viewer understand how long the necklace is, how flexible, where it lies on the neck.
Example: the first two photos I took on white muslin, and they are really beautiful. The colors show well, and the photos give a wonderful idea of the textures and the beauty of the beads.
However, they don't show what it would look like on a person. Here's where the necklace stand comes in:
Isn't that much nicer? It isn't an attention grabbing photo, and you can't see the details of the necklace, but for someone who is interested and wants to know more about the necklace, it provides that information. Without it, the viewer might be left wondering how to tell what it would look like, if it would be too short, too long, etc.
I got two of these stands the other day from Michaels, for 7 or 9 dollars each (I forget). I got this one in a neutral linen, and one in black velvet. Although the black really makes some of the colors pop, it's kind of impossibly to photograph without looking dusty. So that one's on hold for now, until I figure it out.