The levers are a bit more abstract than the other instruments. The lever controls something in the synthesizer called a “MIDI cc”. It’s a parameter which defines part of the sound. Usually it’s assigned to either pitch or a high pass or low pass filter.
An acoustic analogy is a trombone. A trombonist controls the pitch by where he/she positions the teloscopic slide on the instrument.
Electronic dance music tends to have a lot of action with pitch, filters, what have you, so SitM felt it was necessary to have the musicians controlling this aspect of the music rather than the computer. Both Mr. Technology and St(br)ainless have a lever at their stations.