Tag watching is not a whitelist. Tags can be in 3 states: ignored, watched, and normal. Ignored means that such tags don't appear on your home page, ever, and any list of questions which include such tags will have that question grayed out. Watched tags means that the questions will appear highlighted on your home page (and on Stack Overflow, the filtering algorithm will filter out fewer questions with those tags). And normal is... normal.
The main thing that tag watching does is give you easy access to the tag itself. You see it on pages that list questions, so you can quickly select a tag and see the most recent questions on it.