xfx is useful for defining operators that do not evaluate to a value that has the same type as its operands.
1 * 3 evaluates to
4, which is of the same type as
1 < 3 evaluates to truth or falsity, which if of a different type than
xfx prevents expressions like `1 < 2 < 3` that are nonsensical in Prolog from being written down.