Six permutations of the same three operations. On the left, class methods that mutate shared fields produce different results depending on call order. applyCoupon before addItem calculates the discount against a stale total. calcTax before applyCoupon taxes the full amount. The class cannot protect you from calling methods in the wrong order because every method reads and writes the same mutable fields.
On the right, pure functions produce identical results regardless of order. Each function takes its inputs explicitly and returns a new value. No function can see or affect another function's computation. The composition is order-independent because there is no shared state to corrupt.
This is the property that ensures correctness. It eliminates the entire class of bugs that arise from "this must happen before that." It is more important than encapsulation, more important than polymorphism, and it is the foundation that makes safe parallel execution trivial.