当返回类型实际上是错误时，为什么需要返回一个指针？

I am reading the article Error handling and Go, and don't quite understand why a pointer (&errorString{text}) has to be returned when the return type is actually error?

My understanding is error is an interface, and errorString implements the interface, therefore, return errorString is also okay (but it is not the case).

// New returns an error that formats as the given text.
func New(text string) error {
    return &errorString{text}
}

errorString implementation

// errorString is a trivial implementation of error.
type errorString struct {
    s string
}

func (e *errorString) Error() string {
    return e.s
}

Because error interface for errorString is implemented for a pointer (func (e *errorString) Error()), if it was implemented like below you would return the value directly:

func (e errorString) Error() string {
    return e.s
}