Foods are subjected to a very high temperature in the deep-fryer. A protective coating prevents a hard, unattractive skin forming on the food while allowing the inside to cook slowly and thoroughly.
If deep-frying is done properly, the coating also creates a seal that stops a food's flavor from tainting the oil, so it can be used again. Sealing has the added advantage of preventing any dangerous spluttering or bubbling of hot oil, which can happen if food juices leak out.
** Asian Recipes