Not, I hasten to add, in order that they might therefore be stopped... That is a different matter of probabilities and choices involving multiple 'other people' as well as God's plans.
The real reason is essentially twofold - first that we be not misled; and secondly that if our discernment cannot detect the ruling evil, then we are ourselves deficient.
I mean that when a person cannot detect evil when they encounter evil; then there is something seriously wrong with them. Often this wrongness is self-inflicted (people do detect evil - at first, but over-ride their own alarm systems, ignore the evil - and end-up denying evil, hence serving evil).
But if we do not recognise the evil of the powers of the world, then they will divert us away from God and from Jesus Christ. In a thousand and more ways; evil powers will consume our attention, effort and time in thinking/ feeling/ doing things that are not what we ought to be doing.
Yet, knowing the evil of the powers of this world is only a beginning. It does not make us right, of course.
But it is - for most people - impossible to function as we should function in this world (which is, after all, a world in which a single, vast, linked, totalitarian bureaucracy and social manipulation system penetrates everywhere and into all human affairs) unless we understand that the world is against us: that what the world wants for us is the opposite of what God wants for us (including that the world wants us to deny the reality of God).
This is the flipside of economic/ political/ legal globalisation and the vast expansion of the mass/ social media... In the past people didn't need to know much about the nature of powers of the world (which were local and limited in scope, anyway). Now we pretty much have-to.
It used to be possible to be 'neutral' about the motives of the Big Powers and Top People, or at least to ignore the issue... But now we can only choose between regarding them as overall-Good, or net-evil.
And upon that decision may hinge our eternal destiny.