Animating the 9th Amendment was a concern that some may interpret the omission of some rights in the Constitution to suggest that those rights are not protected. The 9th Amendment, in some senses, rebutted the expressio unius cannon.
But, some may ask, shouldn’t the Constitution be read against an implicit expressio unius cannon, like most statutes? Alexander Hamilton in Federalist 83 demurred, arguing that this cannon should not be applied to the Constituiton:
Even if these maxims had a precise technical sense, corresponding with the idea of those who employ them upon the present occasion, which, however, is not the case, they would still be inapplicable to a constitution of government. In relation to such a subject, the natural and obvious sense of its provisions, apart from any technical rules, is the true criterion of construction.
Expressio unius is applicable to statutes, but not Constitution, says Hamilton. Thus, it seems we did not need the 9th amendment to negate his cannon with respect to the omission of certain rights. Like I said, every time I read the Federalist, I see something new.