The rule is not a strict requirement although broadly speaking a trade mark owner is required to enforce their trade marks in relation to third party use which is likely to confuse the public.

The rationale is that trade marks are designed to be a badge of origin & to guarantee to consumers that the goods are from a particular entity. If I do not show due regard for maintaining that distinction by taking action against third parties making confusing use of an identical/similar mark then I should not have the right to continue making use of the mark, or at the very least could be said to have consented to the third party use and cannot subsequently stop them.