In my view it makes the code extremely difficult to understand when someone other than the original author tries to read/modify. The side effects of changing a

In my view it makes the code extremely difficult to understand when someone other than the original author tries to read/modify. The side effects of changing a global variables value is very difficult to glean from code. It is as if some inputs to a function are getting passed to it implicitly, and it isn't obvious what value it has, who has set it, and what effect will be produced if you change its value.