A statement is a block of code that doesn't return anything. An expression, on the other hand, returns a new value. Since statements produce no results, the only way for them to be useful is to mutate state. And functional programming tries to avoid mutating the state as much as possible. Theoretically, the more we rely on expressions, the more our functions will be pure, with all the benefits of functional purity.
We've used the if expression many times already, so one of its benefits should be clear: it's less verbose and, for that reason, less error-prone than the if statement.
