Why is it necessary to use at least one electromagnet in a motor instead of using only permanent magnets?

Because as you might already know electromagnets use a conductive metal and electricity to make the said electromagnet. So what do you get if you use the electromagnet and move it through coiled wire. The magnetism makes a electrical current thus making your motor.

I hope that this was helpful for you. :3  

A. they hold atoms  together with a molecule.its a force that exist between molecules. hope this .

Amplify and switch electrical power and electronic signals