Have you ever made a purchasing error in the past? Or do you have games attached to your Steam account that arrived their via a bundle or something similar, and do you hate looking at them every time you boot up the client? Do you consider them eyesores, constant reminders of your folly, mistakes that follow you around like a bad smell?
If that's you, then rejoice, because a new feature coming in the Steam beta client allows you to hide games in your collection that you don't want to see anymore. Hurrah!
On top of the new hide feature, there's UI improvements and other tweaks / changes that come with the latest client. As it's in beta, you have to opt in to receive these new features now, or you can simply wait until it has been bug-tested and deemed fit for public consumption like everybody else.