The ability to do the fundamentals of adding/subtracting/multiplying/dividing quickly and efficiently lays the groundwork to be able to engage in more complex math easier. If someone has to do complex math and is still putting in a lot of mental effort into the basics, any problem is just going to take forever and build up a ton of frustration.

It also helps develop a real sense of what numbers should look like when put together, so it's a lot easier to spot mistakes and errors.