Mental math is essential for having strong numerical fluency, for estimation, and for reasoning about many systems. Those skills are incredibly useful for thinking critically about the world.
To a certain point. Basic arithmetic is important but the ability to calculate large square roots or multiply multi-digit numbers is not very relevant when you can trivially calculate them on your phone in seconds.
Mental math is essential for having strong numerical fluency, for estimation, and for reasoning about many systems. Those skills are incredibly useful for thinking critically about the world.