I came across the idea of using caulking as a material for modeling many years ago, but I can't quite remember where I originally seen it. I finally gave it a try on the Nurgle Warhound titan to give it the detail and sickly decaying fleshly look. Caulking is very good at making organic shapes, but it needs a base shape to cling to as it isn't very stable on its own.
In the example of the Warhound, plasticard, cardstock, PVC piping, RG6 cable, and even nails were used to create the base model, and this was supported by greenstuff in strategic locations. Greenstuff was also used to make any details that were not "organic" or for a solid base around which to build up the caulking. I then added the caulking.
The advantage is that because of how easy it is to get started with caulking it works very easily and it is several times cheaper than green stuff as well. I used a squeeze tube of the stuff to fill out the detail on the model, then shape it with the sculping tool. Caulking has a habit of clinging to everything (as its designed to do) so you need to keep whatever you are using wet to avoid buildup. If you get some stuck on your tools you should wash it off immediately because once it dries it will be very difficult to remove.
After priming, caulking is fully paintable as well!
As a final note, caulking never quite becomes fully "hard". It always remains slightly like rubber, so it ends up differently and can't really be used to create models from scratch. It is quite durable however and is a big time saver.