From the NY Times:
Here is how the primary runoff in New York was supposed to work: To avoid nominating a politically vulnerable fringe candidate for the general election in November, a second election would be conducted between the top two vote-getters if no one received at least 40 percent of the vote.
Here is how it has worked since the first runoff, in 1973, in a city where enrolled Democrats outnumber Republicans by about 6 to 1: More often than not, the citywide candidate who finished first in the primary but failed to win by 40 percent won in the runoff and then went on to easily win the election in November.
This year, too, whoever wins Tuesday’s runoff in the public advocate and comptroller races is virtually assured victory in November.
Still, the city’s Board of Elections will spend about $15 million to mount the runoff, and the four candidates competing in the election can legally spend a total of nearly $8 million. Fewer than 10 percent of registered Democrats are expected to vote.
So the question among some government watchdog groups is whether runoffs, at least in New York, are superfluous.