HP will often use more water, actually. HP is also quite a bit more work, IMO. The only reason to do it is to use things that don't work well in CP like stearic acid or really accelerating fragrances.
Another important thing with HP, is that you can decide what is going to be soap and what is going to be superfat. I follow some HP-soapmakers on Youtube, and it seems like that is the most important reason behind them doing hot process instead of cold process. They make soap of cheap fats and use the expensive butters as superfat, for example.
I have also read that castile soaps gets better done HP than CP. But I have no clue if that is the case or not.
Another reason, apart from accelerating fragrances, is that they don't run the risk of their scents morphing or fading because they are eaten by lye. I understand that they can use citrus essential oils in HP as well, which will disappear in cold process. But I'm not too sure of that, I might have mixed things up. They also don't have to worry about trace, they just stickblend everything to thick trace and that's it. Partial gel does not exist in HP. No soda ash either. But of course there are problems with HP that those doing CP don't have, like that the soap suddenly can volcano out of the crock and end up all over the countertop + the floor. It can start to solidify before you have got it into the mold (I have experienced that).
How much work HP is depends on their method of making HP. Stickblender hot process (SBHP)/countertop hot prosess is very fast. I have done it myself, and it became quite a lot of work because I messed it up, did not have the right equipment etc, etc. But for me the problems started after saponification. But going from oils and lye to soap, that went as fast as a lightening! For those who are experienced enough to know what to do and what not, and actually can follow recipes without experimenting with everything, they can make HP in a hurry. I of course messed everything up with my experimental methods, that I have learned is not too clever to start with for an absolute beginner in soapmaking.
I think there are many reasons to do HP. I'm not sure though if I will do it again soon, since I now have a recipe I like (I just have to boost the bubbles a bit and it is close to perfection), and that recipe does not have a particular oil going to superfat or anything. I will see when my fragrance oils arrives from Australia, if some of them requires HP, if not, I will make my recipe CP. But I do like HP, it is quite fun when the volcano action starts. I think I have done 3 HP soaps, and two of them ended in the garbage. One was a milk soap, and I had no clue then that the horrible ammonia smell is normal, so it ended up in the trash as a total failure, even if it most likely was just fine. I have not tried the slow HP method, cooking in a crockpot for hours. That seems totally boring and absolutely unnecessary, since it just is to warm the oils to a higher temperature to begin with, dump in hot lye and your HP soap will be done in almost an instant. I don't see any point in doing slow HP, watching over a crockpot for hours when HP really does not take that long at all. If you have 10-20 soaps to make at once (like Essential Soaps on Youtube), well, then it makes sense to use the slow HP method. The fast method would be almost impossible for more than 1 soap at a time.
