The Band Compressor was designed for fixing and enhancing by compressing and trimming a specific range of an instrumental or vocal track. It makes the track tonally more consistent and controls abberations in crucial frequency ranges.
The Band Compressor is a sort of dynamic EQ that works only on a selected frequency band.
The bands are:

LOW - shelving response below about 120Hz. Perfects for bass, drum subgroup, vocals (it controls pops and proximity effect) and overall mix.

L.MID - peaking 250-300Hz, the "hard", muddy low mids that usually are very demanding to be controlled

LOWS&HIGHS - both lows and highs, the "contour" curve. To be used as an automatic "loudness" control

H.MID - about 3kHz peaking, the presence band. If excessive boost is applied it gives unwanted strident and harsh quality causing listening fatigue, compressing the band avoids the effect

HI - shelving response from 5kHz up for brightness control. Good on everything, especially on vocals (it lets to combine HF boost with a de-esser in one process), guitars and drums overheads, as it allows to brighten the drums without cymbals all over the place.

AIR - shelving response from 10kHz upward with a steeper slope. Designed for overall mix processing.

Only the selected band is controlled (cut, boosted or compressed), the rest of the frequency spectrum is unchanged and remains intact.
The direct path can be muted with the DRY CUT switch. It lets to listen the signal that is being processed and can be used for parallel processing as well.
The band filters are passive apart from the L.MID filter and the filtered signal is compressed by a high quality FET compressor with ratio and attack/release times carefully selected for each band.
If the compression knob is on 0 then no compression is taking place, the unit is working as a one band EQ with the range from -10dB to +12dB.
In contrast to multiband compressors, the BC is very easy and intuitive to use and its minimal signal path topology yields less audio degradation.