In general, the BandPass filter is used to isolate one type of 'ess'. The BandPass filter 'looks' at a narrow range of frequencies.
The 'ess' sound in "Sweat" carries most of it's energy around the 6kHz mark (male voice).
In "Sugar" most energy lies around the 4kHz mark (also male voice). Use the BandPass filter if you only want to reduce one 'ess' and not the other.