The prosecution wants to make sure the process by which the evidence was obtained is not truthfully presented, because, as often as not, that process will raise questions.
I grew up in the church, with traditional hymns, but at the same time I was beginning to listen to pop music, the mid-60s, The Beatles, which had just as much influence on me as those hymns did. Then the hippy stuff like Pink Floyd started to raise questions about how I lived my life and the world in which I lived.