Learning long division improves your ability to understand algorithms in general.  It also improves your ability to estimate.  These are both very valuable.

The person who becomes good working with numbers in their head actually spends much less mental effort on a problem than others even if calculators are allowed.  If they're in a meeting and someone asks a number related question (how much of this, or how long until that), they can often give the answer or a close estimate while others are still thinking about how to turn it in to a math problem.  They have a much better intuitive feel for things and can even subconsciously quickly make computations.

I do realize that some people have an easier time and some a harder time developing this part of their brain.  But for everybody, the more powerful their brain is the better.