Think of tags as like a book index where you find words and which pages they are on, the same principle applies here. Although it's taken a stage further so it might not be the word, but the subject, so for example if you're looking for things to do with fractal gradients for example. A tag may bring back posts with gradient files, as well as posts that discuss gradients, or tutorials about them.
The spherical variation reflects the plane across the unit circle (the circle with radius 1 centered at the origin). Mathematically, this is called “inversion in the unit circle”. This is illustrated with the following contrived example, which uses five transforms with svg_wf to create five purple creatures (the default svg_wf graphic) in various locations.