A heat gun is a device that generates an excessive amount of heat for testing and repair. For example, an engineer could point a heat gun at an IC, or other hardware and test what it would do when it overheats. Typically when hardware overheats or works hard and generates heat, safety shutdown procedures are activated. A heat gun is an excellent way of generating heat and testing these situations.
Heat guns are also used while repairing mobile phones, tablets, and other computers that use epoxy or other glues instead of screws. Heating up the glue helps in removing the part or glue from the part so it can be replaced. Once the glue is removed or the epoxy bond is broken, you may need to apply more.