Great Shane Goldmacher in the NYT:
For decades, New York seats have traded hands this way in what amounts to one of the last, most powerful vestiges of Tammany Hall-style politics in the state. Election laws here grant politicians and local political power brokers vast sway in picking candidates when legislators leave office in the middle of their term — whether they retire early, pass away, depart for another job or are carted away in handcuffs.
The rules are a crucial part of what empowers party bosses in a state that regularly outpaces the nation in corruption. They encourage ambitious politicians, even the most independent ones, to pledge fealty to county political leaders, lest they get passed over if and when the time comes for possible advancement.