Some sectors aren't "productive" on their own but still necessary. Railways basically never pay for themselves by fares paid by (mostly) workers out of post-tax income, but the value they bring to the economy by enabling economic activity is worth it.

Roads, education, healthcare, much of these things would either be unprofitable or unaffordable if they have to be self-funding. But they bring social value even if they don't make their own shareholders rich.