Silicone, rubber and latex molds (or flexible molds in short) are mainly used to create what is called novelty candles, candles that can take all sorts of asymmetric shapes.
To list just a short sample of what can be achieved using flexible molds, I give you dinosaurs, witches, pumpkins, smurfs, skulls, human hands (showing a variable amount of fingers), buddahs, bears, ducks, fruits, unicorns, male and female attributes, rabbits, Santa's, Easter Island heads, ... You name it: it exists.
The main advantage of flexible molds compared to their metal or polycarbonate counterparts is the endless variety of shapes as well as the level of detail they offer (the Triceratops' horns, Santa's nose). The mold being flexible, it allows you to unmold the finished candle without losing any of its details.
Silicone and rubber molds usually look like a cylinder on the outside (picture above left) and are freestanding. Latex molds, on the other hand, have the shape of the item they were originally created from. Their biggest drawback is that they won't stand by themselves when you've poured the wax inside them so they require a little bit of DIY. The base of a latex mold (see picture above right) has a kind of rolled up "lip" that is larger than the section just above. Based on the shape and dimensions of that lip, you can cut a hole in a piece of cardboard; make sure the hole is large enough to let the body of the mold go through but small enough for the lip of the mold not to. Then all you have to do is find a way to "suspend" the cardboard with the mold stuck upside down into it (a plastic container of the right size is perfect) and you now have a nice little holder for THAT particular flexible mold (you will soon realize that the size and shape of the lip is never the same).