To get a bit technical: In a way, it is a “refuse” of the chemical bonding between the three fatty molecules of oils with the alkali molecule (potassium hydroxide), which leaves the fourth component of the oil molecules (glycerol) unbound and easily extracted from the soap.
In industrial soap making, glycerine is reclaimed from the soap paste, purified and sold as a separate product.
At the end of the soapmaking process, the potassium hydroxide (a processing aid) has been used up and the natural handmade soaps contain all the available glycerol molecules (glycerine), which contribute to its mildness and skin-friendliness.
Traditional handmade soaps all have a pH of between 8-10. Due to the high pH, it contains no preservatives, but should you wish to add any botanicals like aloe vera or herbal extracts it is advisable to add a preservative. This is not needed when adding essential oils.
There is a modern misunderstanding that a chemically based soap substitute is safer & milder on the skin, as it has a lower pH and that a natural & traditionally made soap from vegetable oils is harsher because it has higher pH. The fact is that the pH has no bearing on the mildness or harshness of a soap.
This misunderstanding started with the incorporation of chemicals into the soapmaking process, when during and after the 2nd world war, vegetable oils and fats were scarce.
To create a marketing edge over the traditionally made soaps and as these chemicals were naturally acidic in nature, the myth was created that soaps with a lower pH were more gentle on the skin that those with a higher pH.
Soaps manage to cleanse the skin as dirt and oils have a tendency to migrate from a surface of a lower pH (the skin has a pH of around 7) to the higher pH of the soap and it is a fact that skin conditions of thousands of people have improved after changing over from chemical soaps to natural soaps.