Question

why is friction know as necessary evil​

Answer 1

Friction is known as a necessary evil because it has both harmful and beneficial  effects

First of all, we will understand what is friction to get a better understanding of the question above

Friction is a force exerted by a surface against the motion of a body across its surface.

Now it is known as a necessary evil because Basic activities like walking and writing on a surface are possible due to friction without friction we are unable to perform any of these activities. This is the necessary part of friction.

Now let us see why friction is considered  evil

The Frictional force creates wear and tear in the machinery.  As it always resists the motion, so extra energy is required to overcome it.

Advantages of frictional force

1. We are able to walk on any surface because of the frictional force or opposing force it provides to us.
2. We are able to write things on a piece of paper or a surface because of the frictional force it provides  

Disadvantages of frictional force

1. It always resists the motion, so we require more energy to overcome it. Wastage of energy.
2. It decreases the overall longevity of the object(machine parts) etc.

This is why friction is considered a necessary evil.