Immutable storage

Garbage collectors simplified mutability considerably and, as a consequence, many of these languages don't need a specific modifier for mutability. While this garbage collection mechanism comes at the expense of runtime with frequent cleanups, not having to care about making variables mutable is very convenient. It lets developers focus on the logic they are implementing instead.

So why did Rust (and many functional languages) bring back this concept?

States and reasoning

The state of an object is essentially the current values that its fields have at any given time. Changing this state is done by the object itself via messages on defined behavior (called methods) according to object-orientation principles...