Klout is really just an arbitrary reputation ranking system, just like the Bartonphink system. However the difference lies in what people conclude from an individual's ranking. People use Bartonphink to rank someone on their pertinence to bartonfink. People use Klout to "measure someone's online influence". This is extraordinarily important to businesses looking to do marketing, sales, customer service, etc.

The point I think you are missing is that Klout, and other ranking systems, can give a quick and objective assessment of someone's value or ability in a certain area.

Here's a list of ranking systems and what they indicate:

Klout -> online influence StackOverflow -> programming expertise FourSquare -> loyalty to a business Credit Score -> fiscal responsibility HackerNews Karma -> ability to provide interesting and pertinent points of conversation