What does the loop in the magnetic fields do? why?

Answer 1

Basically, the "loop" is a return path. It does two things: it allows the creation of an opposite polarity magnetic pole, and it follows the "conservation of energy" theory.

Think about it like this: if you were pouring water from one bucket (like a north magnetic pole) to another (a south magnetic pole), eventually the first bucket would run out of water. Then what do you do? If there's a return line with a pump hooked up to the second bucket, though, pushing water back up into the first bucket, you can keep pouring indefinitely.

With the Conservation of Energy, there has to be some way of refilling the bucket. It doesn't matter what form - energy is basically the capacity for doing work. Forms of energy include thermal, mechanical, electrical, and chemical. Energy may be transformed from one form into another.

Answer 2

The loop I believe you are referring to are flux lines. They don't do anything, they are just the way magnets work. These lines have an electromagnetic charge.