In any election with more than two candidates, if no candidate wins a majority, there is no guarantee that the plurality winner, or even the top two pluralities winners, are the two candidates who are favored by a majority of voters. This is because two popular, but similar candidates, can and will take votes away from each other. The more candidates that are running, the larger the risk of similar candidates taking votes away from each other becomes. Giving voters a ballot on which they can record the ranking of their first, second and maybe third choices can eliminate this unnecessary and unmanageable risk for candidates.
A primary runoff that requires a separate election costs more for both candidates who need to campaign until the runoff and for the taxpayers who have to pay for the cost of another election. Moreover, the voter patience, interest and turnout for runoff elections is historically significantly lower.
Some other significant benefits from a same-day runoff voting system are: When candidates need to have both first- and second-choice votes, candidates will form coalitions on certain issues and then explain both their similarities and their nuanced, but still complementary, differences. This means campaigns have an incentive to be more forthright.
The fact that campaigns will have less unnecessary and unmanageable risk and also be more forthright will attract more good leaders to run. Also, more female leaders will run and will be elected, the agendas will expand, and voter interest and turnout will increase significantly.