I don't use my PP2 anymore (got the MC403, way better), but some people find it useful to get a muddier sound from overdrive pedals.
Some people like clean headroom (12v-18v).
Some like muddy dirty sound (<9v).
It gives the sound of a dying battery. However, some pedals don't function at lower than 9v. It should really only be used for boosts/overdrives. It might work for an analog delay to get a grittier sound.