Clearly the judges weren't as interested in hearing the truth as they were in getting rid of the sin of witchcraft. Danforth, in practically the same breath, told Giles he had to follow the rule of law and that he didn't care what the law said. The test tells us the judges and the girls eat their meals together--a practice unheard of in this world, one guaranteed to skew the proceedings and unbalance the scales of justice. If Giles Corey, the most litigious man in the entire region, is not able to get satisfaction in this court, no one else is going to get it.
The fraudulent nature of the trials is what is explored through the play. One of the most powerful elements that are brought out is that if individuals in the position of power seek to abuse their control, trials can turn out to be exercises in corruption. This helps to underscore the potential to abuse the legal system, and the need to protect it from such influences. Giles Corey's interruptions are done in the name of making right what has been twisted into so much wrong.