Thermal cookers are useful in a different way than pressure cooker.
Pressure cooker help in cooking fast and using less fuel too. It is the most suitable and practical way to cook items that take long time to get cooked softly and properly fast. It also helps to cook multiple recipe and menu simultaneously in one vessel itself, thus saving a lot of time.
Thermal cooker on the other hand is suitable when there is sufficient time at hand but you want to save the fuel and also use the time for some other work. It works on the heat retained inside the vessel in the cooking medium. It is a slow and steady cooking. It is suitable to cook grains and dry wholesome pulses. Thermal cooking adds to the taste of cooking as the cooking is slow and in normal pressure not instantaneous by pressurising. As there is no high pressure applied, the food items do not lose shape nor lose their nutrition. Thermal cookers are safe too because they are not pressurised and you can open them at your will, and it will not burn your hands like steam from pressure cooker.
But thermal cooker can be used for time saving also if properly planned.
For example, the brown/red 'Matta' variety of rice, commonly used in Kerala needs a longtime for cooking under normal pressure. That will be a waste of fuel also. In Thermal cooker, it uses very less fuel as once the water-grain mixture is well boiled, it is allowed to remain in the thermal cooker with the internal heat by switching off oven/stove. The thermal cooker is opened after a few hours, and by then the rice would be well cooked for use. ( It is because of this reason that Thermal cookers are more popular in Kerala.)
The timing can be adjusted by experience, and as per the item and our needs.
So those who have to leave early in the morning can use thermal cooker to boil the rice at night before going to bed and turning off the gas/stove, keeping it so till they wake up next morning. On waking up the item will be properly cooked and properly warm for use.
I had used 'Chakson Thermal Cooker'. It was useful to me as we were using the 'Matta' rice variety. Chakson is made and marketed by the 'Anna Aluminium Group' in Kerala. In local parlance Thermal cooker is also called "Choodaarapetty"or heat retaining box. You can get more details like cost etc in their web site : kitexlifestyle.com and search for 'choodarapetty'. or you can directly search for 'Chakson thermal cooker'.