Statistics and Modelling
A long time ago, in the university, I know a lot of things: Matlab, SPSS and Excel. And I know some of my friends in university that very good on those software. I see the software interface, it was interesting tools. But I never use it. In the early time of my work, I also know a lot of things: ACL, Monarch. But I also never use it.
But you know, we never know when things -in this case tools, would be useful.
It also goes with Karl Person (Correlation) and Gauss (Normal Distribution), I know that some of my friends are very good at quantitative analysis, but so far I solved my problem mainly using qualitative analysis.
Stackexchange for reference.