Failure to do so would enable a voter to both cast an extra vote for and an extra vote against, allowing voter fraud; by design, a for vote cannot be distinguished from an against vote once cast, so this multiple-vote fraud could not be detected until the final tally verification (and maybe not even then), and it cannot be corrected at that point or even traced to a specific voter.
Failure to prevent multiple voting would allow a voter to cast two votes, one for and one against, leading to voter fraud; by design, a vote for cannot be distinguished from a vote against once cast, so this type of multiple-vote fraud could not be detected until the final tally verification (and maybe not even then), and it cannot be corrected at that point or even traced to a specific voter.