Danforth summarizes how in a normal case of crime, you have actual witnesses to the crime, eye-witness testimonies of what was done. For example, multiple people can get up and testify that "Yes, I saw that man rob the store". However, in the case of witchcraft, no one is witness to the crime except for "the witch and the victim." Because a witch certainly isn't going to "accuse herself" of the crime, "we must rely upon her victims" to testify against her. Danforth claims that the girls are all victims who are testifying against the witches. For this reason, they should be believed, because no one else was witness to the crime.
So, the victim's testimony is the only testimony that counts, because she is the only one that was there when the crime occurred, the only one who "saw" and "felt" the witch's power.