This technique is mainly used for cooking meat. It gets cooked in fat, on high heat and quickly. This process will ensure a crunchy brown crust (this is because the heat caramelises the sugars and browns the proteins present in the meat). This is not to be confused with ‘sealing’ the meat, as a lot of recipes and chefs claim you are doing. The heat does NOT seal the juices in the meat, which is why meat is dry when over-cooked. Searing adds colour and flavour only.
This can be done to a fillet steak, when cooking rare or meats for stews and roasts, before slow cooking. This step gets often overlooked, but it is this that adds colour and flavour to the finished dish.