“The depressing thing about tennis is that no matter how good I get, I’ll never be as good as a wall.” (Mitch Hedberg) Now, I could say similarly: no matter how good I play an instrument, there’s this computer program that does it better. Seriously, I don’t know what kind of AI they use in Band in a Box, but the program plays incredibly well, in a wild variety of styles.
Here’s how it works, basically. A sequence of chords is input, with its measures. E.g., a simple blues progression (E-A-E-E-A-A-E-E-B-A-E-B). Then, a style must be selected, from a wide variety of Western styles (country, jazz, bossa, rock). And off it goes! Generally, the program builds its different lines (drums, bass, guitar, piano, solo…) following the harmonic progression defined by the chords. It obviously anticipates the next chord, maybe the one after too. It also repeats the parts of the song that are specified (verses, chorus), but does not repeat the same pattern; at least not in the first pass, which is usually simpler.
Two important uses: one may use BiaB as an accompanying band, playing along with it. Another nice feature is the possibility to print out the resulting score (in tablature, too) if a particular line sounds good and wants to be practiced.