I am sure that there are many jobs where mental math makes a huge difference.

I manage a team of engineers, and though pretty much all of them are head and shoulders above me in their specialisation, they think I really know my stuff because I find errors in their work and zero-in on them on the fly. The skill that I have is doing rough approximations in my head. Then from experience: a factor-of-two difference is commonly confusing kg and lb, a factor of 10 - confusing kg and N, a factor of fifty - mistaking degrees and radians (usually in Excel, where radians are the default mesurement), etc... I get a LOT of mileage from this :). If they did the same, their already good work would be even better. And I imagine any calculation intensive job (finance, economics, science, business...) is similar.