Answer:
Because always when we do an experiment, we need something to compare.
For example, if we want to see how much change a given color paint when we add different amounts of white paint to it, we need to have at least one sample of pure color paint, so we can compare it whit the ones with mixed paints.
Well, the case of the polygraph is exactly the same, we need to have a baseline, so we the interviewer can see if for some questions there is a change in the line compared with the baseline.
