"The human understanding when it has once adopted an opinion (either as being the received opinion or as being agreeable to itself) draws all things else to support and agree with it." —Francis Bacon
Confirmation Bias is the human tendency to trust information that supports our beliefs and reject information that doesn't. I can only think of one reason why humans would benefit from confirmation bias: it's easiest to be a happy member of your tribe if it's hard for you to change your belief system.
But the simpler explanation is humans are crazy.
Confirmation theory pays the bills in journalism. From Confirmation Bias « You Are Not So Smart:
Punditry is a whole industry built on confirmation bias.More: Confirmation bias - Wikipedia
Rush Limbaugh and Keith Olbermann, Glenn Beck and Arianna Huffington, Rachel Maddow and Ann Coulter – these people provide fuel for beliefs, they pre-filter the world to match existing world-views.
If their filter is like your filter, you love them. If it isn’t, you hate them.
Whether or not pundits are telling the truth, or vetting their opinions, or thoroughly researching their topics is all beside the point. You watch them not for information, but for confirmation."