The Imacons (like just about every scanner) have an trilinear CCD. This means there's one line each for the primaries, in this case R-G-B. The film is stepped underneath the scan head, and for each step each line is read out. The channels are shifted to combine back into proper RGB before further processing. Each line is clocked out separately, with a different output port. Each of these ports feeds into an ADC; one for each channel.
The first thing that happens to the analog output from the CCD when it gets to the ADC is it goes to an input amplifier. This allows the ADC to have high input impedance, clean up the signal, tweak it suit the quantization (like adjust voltage levels prior to the hold stage where it's locked for the quantization), and so on. Just about all ADCs designed for quality image capture have programmable input amplifiers, using two parameters: offset and gain. The offset is subtracted from the analog signal during amplification, and the gain, well, adjusts the amplifier gain - its multiplier.
If you imagine a histogram captured without offset, that's slightly shifted to the right, then recapturing it with the offset(s) properly set up will shift it to the left. Since there is one ADC per channel the offset(s) specify the black point. (White point for negs.)
If the histogram doesn't extend all the way to the right, then an increase in gain will push it out. Similarly, if it's extending past the end (i.e. it's clipped), then reducing the gain will pull it back in. In other words, this determines the white point (black point for negs). These two points are very similar to how you would adjust them using the levels tool in Photoshop - the transformation is trivial. They can be used for instance to eliminate the C41 orange mask.
Since most noise during a scan, apart from film grain, occurs during hold and quantization, rather than in the transfer from CCD well to ADC or during input signal conditioning, getting the black and white points correct here is a very good idea. Adjusting the black and white points post capture multiplies the quantization noise by the gain factor, and anything that stretches the histogram will need to apply a gain.
When you set the white and black points on the Imacon it adjusts the ADC channels. In fact, they even made a point of this in some information at some point. (But it's been a while, maybe 5 years, and my memory is very leaky.) It's not a given that all scanning software does the same, or that all scanners even support it. But I'd expect any high-end device like the Nikons and Minoltas to work the same.
The light control is useful to adjust for base density of the original. I usually bump it up for transparencies, and set it a little lower for neg (color or B&W). It causes the whole histogram to shift and is needed if the min/max densities are such that the min/max black/white point settings can't contain them. This all gets saved into my film profiles,