A boost pedal can be really useful for a lot of different applications.
It can be used to thicken your sound from the preamp section when placed in front of the amp. It will make the signal fatter and stronger. You can also use it for levelling differences between single coil guitars and humbuckers.
Most guitarist will use a boost pedal to boost significantly the signal and thus raise the volume of the amplifier, in order to bring some some section forward in the mix (i.e. Solos).
The Spark App proposes a boost pedal called Booster. It is modelled after the MXR Micro Amp pedal.