Ever played a piece on a specific guitar and can just take it to the moon? Then you switch to another that you love dearly and just can't quite reach orbit? That's what I noticed about my own playing and had to add an instrument editor and support for tracking which guitars were used to perform a specific take.
If you're lucky enough to have a large collection of instruments (they don't have to be guitars!), this is a great place to keep notes about them too (for grins, insurance purposes, or just to cheer yourself up).
Any instruments defined here will be selectable in the Exercise Editor (as a default instrument for that exercise) or the Take Inspector (the guitar you recorded that specific take with).
If you've already read the Statistics page, you'll remember a mention of "future enhancements" - upcoming possibilities for performance analysis boggle the mind!