Bands are the working vocabulary of the spectrum: sub-bass below roughly 60 Hz, bass to about 250, mids carrying most instruments and voices, presence around 2 to 6 kHz where clarity lives, and air above that. Boundaries vary by who is talking; the concept does not.
Splitting into bands turns one loudness number into a handful of independent signals, which is the difference between visuals that pulse and visuals that play. Kick energy can drive structure while hi-hat energy drives shimmer, separately.
Photism analyzes eight bands, chosen so that the parts of a mix producers think in (kick, bass, body, vocal presence, air) land in different slots and can move different things.