Consider this scenario:

Company A has a trademark. Company B creates its own product (perhaps trademarked as well) which is somewhat similar to Company A's trademark but in a sufficiently different product space that a lawsuit by Company A would seem potentially frivolous. Company B then grows its business over several years, starts moving into adjacent product categories, and ends up with a product that now could get mixed up with Company A's. Company A sues Company B, but Company B argues that it has invested a lot into its own brand, and Company A should have been policing its brand years ago to make sure Company B doesn't end up investing in a brand that Company A would end up considering infringing down the road.