Stop words are the high-frequency structural words that hold sentences together — the, and, of, to, is. Removing them leaves the terms that carry the actual subject matter.
Why remove them at all
They dominate any frequency count without telling you anything. In almost every English text, the same handful of function words occupy the top ten positions, burying the words that distinguish one document from another.
What is left behind
Nouns, verbs and modifiers specific to the topic. That residue is a rough but useful summary of what a piece of text is genuinely about.
What it is not for
Stripped text is unreadable and should never be published. This is an analysis step, not an editing one.
Try it: Remove Stop Words on SeoWolf's Notepad.