Julia 1.0 Programming Complete Reference Guide

Why are constants important?

It's mostly about performance. Constants can be especially useful as global values. Because global variables are long-lived and can be modified at any time and from any location in your code, the compiler is having a hard time optimizing them. If we tell the compiler that the value is constant and thus that the type of the value won't change, the performance problem can be optimized away.

Of course, just because constants alleviate some critical performance problems brought about by global variables, it doesn't mean that we are encouraged to use them. Global values in Julia, like in other languages, must be avoided whenever possible. Besides performance issues, they can create subtle bugs that are hard to catch ...