Control of knowledge is control of action
If you control what people know, you control what they believe. If you control what people believe, you can control what they do.
Control
Someone who believes, for example, that X is a deadly threat is likely to strongly prefer a candidate who offers any plan (however dubious) for stopping that threat over any other candidates who focus on other things. If the reality that X is not actually a problem but Y and Z are, then a lot of people may end up voting against solving those real problems in favor of solving the imaginary one.
Divide and Conquer
You can also prevent such believers from having meaningful dialogue with others who do have an accurate understanding of reality. Imagine trying to have a sensible discussion with someone who argued thusly:
My esteemed opponent advocates for increased usage of solar panels as a source of energy for our great nation -- but is she not aware that the suction of light-energy from these panels tires out the horses that daily pull the sun across the sky's firmament? Worse yet, it is well-known that the glare from these panels may cause the horses to panic and rear up in their tracks, risking unimaginable disaster to all life on earth! Clearly my opponent has not considered these, the scientific facts in the matter, or else is so bounden by her financial ties to the multi-million-dollar solar panel industry that she would eagerly sacrifice us all to the inferno of her ambitions!
Nobody today would actually hold such beliefs, but they are no more in touch with reality than many arguments repeated frequently in public discourse.
There is simply no common ground for discussing the nature of reality with those who believe such things, because their beliefs are based on received knowledge rather than evidence and they are generally strongly averse to any suggestion of re-examining the available evidence.
Thus we are divided into a "reality-based community" and a "perception-based community" – and a house divided against itself cannot long stand.
Notes
Something needs to be written about this Chomsky quote as well -- i.e. restricting the range of positions that people are aware of also restrains the range of opinions they are likely to consider.