In small programs, this might not be an issue. ... Global variables also make your program less modular and less flexible. A function that utilizes nothing but its parameters and has no side effects is perfectly modular. Modularity helps both in understanding what a program does, as well as with reusability.

Why are global constants bad?

Global variables are problematic because they introduce largely unnecessary dependencies across modules. These dependencies make it harder to debug problems and reuse code.

What is the problem with global variables?

Using global variables causes very tight coupling of code. Using global variables causes namespace pollution. This may lead to unnecessarily reassigning a global value. Testing in programs using global variables can be a huge pain as it is difficult to decouple them when testing.