The Landau Boost is true bypass with EXP capabilities that can be used to attenuate the boosted level (set by the level pot on the pedal itself) by raising or lowering the foot-pedal on the EXP. The EXP (when connected), takes over the level pot on the pedal itself and re-calibrates the pot taper on the EXP so that it interacts in real-time with the boost to adjust your output level from whatever you've set as the max output (in the toe-down position on the EXP), all the way back to silent (in the heel position on the EXP).
This allows you to always be able to adjust your output in the mix for soloing or for just unity gain swells. The benefit of this over a volume pedal is 1) you can boost beyond unity gain, up to +16dB over unity, and 2) you have no sonic compromise with using the EXP as the volume pedal, as the EXP has no influence over the 100% analog circuitry of the boost. No tone suck as much volume pedals have.