When two or more objects use the same piece of mutable data, they all have the ability to break each other in ways that can be hard to debug. If the shared data is immutable, however, these objects can not affect each other, and are effectively decoupled.
More features equal to more bugs, more complexity — uncovered cases and unexpected behaviour. In software engineering, we call it a technical debt. One of the working solutions to reduce technical debt is Code Review.