The operator solves the problem with numeric overflow that you get with subtraction: if you subtract a large positive number from a negative that is close to INT_MIN, you get a number that cannot be represented as an int, thus causing undefined behavior.
Although version 3 is free from this problem, it utterly lacks readability: it would take some time to understand by someone who has never seen this trick before. <=> operator fixes the readability problem, too.
This is only one problem addressed by the new operator. Section 2.2.3 of Herb Sutter’s Consistent comparison paper talks about the use of <=> with other data types of the language where subtraction may produce inconsistent results.