here's a better question to ask: why should anyone need to justify their own existence?
Who really benefits from that mindset? Who benefits from systems that force people to justify their existence in some way? I'm thinking about economics, and politicians making welfare recipients jump through hoops that cost money more than they save to "earn" basic necessities, and the implications for people with less-understood disabilities and older adults with dementia and people who are marginalized in other ways, and how all of this fits into a world where tech companies want to replace everyone's jobs and hobbies and relationships with AI. (Granted, I doubt LLMs will ever be reliable enough to fully replace humans in some things, and some people will always want relationships with real humans, but there's plenty of other tech being developed out there too.) Everyone on Earth might have an "unjustified" existence as far as society is concerned someday, if we continue with this mindset.
To answer the second part of your question, though, about why should we care: because we are all fellow humans. I think others here have answered that part in more depth.
another thing: I think most people answering here aren't thinking about how it would feel to have the statistic they're choosing over their own heads.