One big problem with mutation is that it makes it too easy to violate many good design principles, e.g. modularity, encapsulation and separation of concerns.

Because any piece of code that holds a reference to a mutable variable is able to, at a distance, modify the behavior of a piece of code that uses this mutable variable.

Conversely, a piece of code that only uses immutable variables, and takes as argument the values that may need to vary between executions, is isolated against having its behavior changed at a distance at any time.