Flagging political threads isn't stopping that either - that spillover is what flagging comments and downvotes are already for, and moderators detaching off-topic tangents.

But what hiding stories does accomplish is preventing people who don't want to see that content from being angered by its presence, especially when a topic threatens to overwhelm the site, and otherwise takes up most of the comments feed in a heated discussion, or most of the new topics page in the case of a popular story.

Although a more proactive approach would be to better curate the comments feed. It still seems that high traffic or volatile threads contribute a lot of low quality spillover there, and a lot of low hanging fruit for flamebaiting and drive-by comments.