What this video covers
The clip frames a vapor barrier as arithmetic rather than a home improvement.
A crawl space with exposed soil releases moisture into the structure above it continuously. Sealing it with a barrier cuts that moisture dramatically, which removes the water supply for everything growing on the joists and subfloor.
The takeaway
It is preventative spending, which is why it gets skipped. The comparison worth making is not barrier versus nothing, it is barrier versus the remediation bill for the floor structure it protects.
Read the full written guide: Crawl Space Vapor Barrier.