A heat diffuser is used to spread heat evenly to the bottom of your pot or pan.
Almost all automatic cooking ranges have a certain amount of unevenness to the way they distribute heat. This is more so with electric stoves, so a diffuser is a must use when cooking on electric stove-tops, like coil, glass or ceramic cook tops.
Stand alone hot plates, slow cooker machines that run on electric also need a heat diffuser.
A diffuser is recommended for gas stoves too (the ones that burn with a flame). But since these stoves disperse heat more evenly than electric it is not a must use. Based on your experience if you think your gas stove is not cooking evenly then please use a heat diffuser.
If using the pot for long hours of slow cooking either on gas or electric, please use a diffuser. You don’t need to use a diffuser when cooking in the oven. The heat diffusers are listed towards the bottom of the MEC products page.