If you read the blog you would know why this information is useful :)

Web workers make it possible to do parallel computation with JavaScript, and knowing the right number to spawn helps to make sure resources are being taken advantage of. We already have the ability to run JavaScript code on multiple processors, but currently, you can't tell where it's being run. For performance reasons, knowing the number of workers to spawn can make a big difference.

Secondly, it's called an estimator so it's never going to be perfect. It's a tool designed to give you some idea of what is going on.