What is the purpose of the `fa-fw` class?

Fixed Width Icons

I've opened the sample page in Chrome, opened inspector on the "Library"-Element of the fw-sample and removed the fw-class. This messes up the icon-alignment - and that is also mentioned as the purpose of fw: ensuring proper alignment of the icons :-)

Icon size, some icons are bigger, if you use fa fa-fw, the size of the icon will be the same regardless of how big/small it should be.