Could somebody explain variable bit rate for me? What's the benefit of doing this over 192 or 256? Why the shift for different tracks?

How do it know?

VBR is a type of audio compression that allows certain sections of the audio to be encoded at different bit rates. The intricate audio sounds are encoded at a higher rate while the simple sounds are encoded at a lower rate. This is different than standard bit rate encoding, which allows for all of the sounds to be encoded at the same rate. Essentially VBR encoding gives you the best possible sound quality in the smallest file size possible.

what pala said.

VBR focuses on quality.
CBR focuses on consistent filesize.

That'll do it. Thanks.