This minimal setup is not about placing reactors in the most efficient way, but features a mechanism to save fuel cells: All produced steam is stored in tanks whose contents are readable. As soon the steam tank is empty a new fuel cell gets fed into the reactor. In the beginning when this setup is sufficient for all energy needs it saves about 50-90% percent of fuel - later, after using kovarex enrichment process, there's usually more than enough available.
It can still be useful, but won't save as much because a bigger number of steam turbines will drain the steam resources more quickly.
Technically, it's a counter resetting every 10 seconds. If the counter and the amount of steam is equal zero, the inserter (stack size = 1) gets triggered.