Assuming you want to know what that option on the top right thread section does:
Is used to hide (a) thread(s) you don't want listed when you browse the forums.
Such sub-thread(s) is(are) not shown to you when you enter the main forum(s) where you have ignored them.
If you're asking about why allowing people to ignore threads they don't like and/or how ignoring threads is useful for site owners and users; then you're in the wrong forum, that question belongs in the off-topic or general section.