If you find a cancerous growth, you remove it before it taints the rest of its host.
Or to put it another way, there is an open question which manuscripts are correct. And we do not just have two types of manuscripts, the KJV type and the NIV type. There are many manuscripts with various differences. Most are not major. Some are.
Now were the verses that are in the KJV there originally and they were removed by the other manuscripts?
Or were they not there originally and people added them later to support their ideas of doctrine. That is the debate.
In the case of the text about the three witnesses some allege that the text was added after the Trinity had already been hammered out by the councils to support that.
Of course others point to evidence that say it was not.
The point remains that if you have 10 different versions of the same document, it is not immediately clear that the longest one is correct, and the shortest is not, or the reverse.
The people who favor the NIV type manuscript go by a rule that says the shorter reading is preferred, because they see scribes as trying to add things to fit their theology.
People who favor the KJV type manuscript tend to say that it is more likely that someone who is copying a document might skip phrases than add new ones. Or that it is possible those who didn't like a particular doctrine might just leave out references.
There are cases where one can see both might be right. Some have studies examples of manuscripts that are unique in a given reading. In other words, only one manuscript that we have today has that exact reading for a particular verse. By looking at these they can see how that change may have happened. Especially if they just skipped a word, or added something that clearly favors a particular viewpoint.
The problem is no one rule is going to account for all the human possibilities among copyists. Sometimes people act differently.