Why breadcrumb is necessary for deep-frying?
The nature of deep frying, contrary to expectation, is that it is a dry cooking method.

The heat from the oil vaporizes water at the surface of the food.  The steam coming of the food pushes the oil away from the food, and keeps oil from soaking in during the main frying process.  This is also what causes the bubbling.

The consequence of this is that by its very nature, deep frying rapidly dries out the surface of whatever is being cooked.

From some foods, including some meats, this would leave the outside of the meat dry, tough, and rubbery.   A coating of breading prevents this by providing a barrier between the hot frying oil and the meat.  

The breading itself also browns, which adds flavor and crunchiness that can greatly enhance the appeal of the food item.  It may also have spices or other flavors or seasonings in the breading to compliment the food.

Still, not all meats have to be breaded before deep frying.  For example, chicken wings are often deep fried plain.  The skin browns and bubbles and gets very crispy.

So, the main reasons for breading are:


Protect the food product
Add brown, delicious crunch
Add flavor through other seasonings