Because it shouldn't be there at all but it is (probably because it was deemed cheaper to leave it on there unused than to actually remove it) but due to a voltage miscalculation it comes on when it shouldn't...this causes a problem. Having it come on when it was never intended to introduces unforeseen consequences into the system.
Think of it like this...What does the human appendix do? Nothing really. It can be removed with little or no consequence because basically it's some vestigial remnant of evolution. Sometimes people keep them and nothing happens their whole life. But sometimes you get appendicitis and this can become potentially life threatening if the appendix isn't removed and bursts. So here you have a completely useless organ which can be perfectly safely removed and you can still live a perfectly normal life but which can threaten your life if it malfunctions...This is essentially the same thing. Having your appendix is no problem as long as you don't get appendicitis. But the mere fact that you have an appendix is one more "moving part" and therefore one more potential thing that can go wrong.
You can make the same analogy with tonsils...a lot of people just have them removed when they are young so that they don't cause complications or infections later in life. They go on to lead perfectly normal and functionally unimpaired lives without their tonsils.
I was certain you were going to mention foreskin.....