Mental math skills are for prosaic daily-life problems, and have almost nothing to do with mathematics as a discipline. (Being good at spelling on its own does

Mental math skills are for prosaic daily-life problems, and have almost nothing to do with mathematics as a discipline. (Being good at spelling on its own does not make one a good writer.) Not needing a calculator at the grocery store to figure out which of some competing products are cheaper per unit mass, or figure out change + tip at a restaurant is useful knowledge.