The AMS1117 on the NodeMCU board is a so called "linear regulator", which means they're not switching.
Let's say you're running at 5V and the output of that regulator is 3V3.
Then you're having 1.7V over the regulator. (5 - 3.3)
At 100 mA, that's 170 mWatt of energy lost.
When feeding from 8.3V, then there is 5V over the regulator.
At 100 mA, that's 500 mWatt lost. (and quite some heat)
When you place a switching regulator in front of it, you're making it a bit more efficient, since that switching regulator has an efficiency of about 80 - 85%.
I don't understand what you mean with your last question.
For the reed switch of the counter i pick a 3.3V and Gnd from two pins of NodeMcu board.
A reed switch short circuits the contacts. So you also need some resistor in series with the reed switch.
And then connect the point where the reed switch and the resistor meet, to a GPIO pin of the ESP.