Question

How is friction both good and evil?

Answer 1

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
If there was no friction,moving objects will keep on moving and will never stop.
But friction sometimes acts as an evil too.The most common example we all have experienced that is the wearing out of shoe soles.

Answer 2

If there is anything which moves forward with respect to something else while in contact with it without any external force acting it's because of good friction. Example, walking human, rolling tires. 

Friction is evil:

If there is external force available then the same friction slows it down. Example, reciprocating piston in cylinder.