It's secret for retaliation plus that. It should be obvious security requirement given all the times in history people got rounded up by the next, opposite person in power. Or politically punished as countries got less vicious. A good, voting system should always protect the voters from retaliation where possible.

It serves both to protect both the integrity of the vote, as well as the freedom of the voter from reprisal. Imagine a less than democratic democracy (if such a thing could be imagined). Dissenting votes, if public, wouldn't be very safe to make.