Push means when the device sends data the App shows it shows it, Reading Rate means the widget must request data from the device.
Push is generally for coded functions on the device, and the data is sent (to the server) regardless if the App is active or not (thus trackable in Superchart).
Reading Rate only works when the App is active, and can be used with direct pin reading AKA, analog/digital reads without needed extra coded functions (e.g. basic “code free” Blynk projects).
I also see Reading Rate most often in the NodeJS examples… like my Clock example… because there is no “loop” I guess one needs to use timers to push the data… but I havent really invested much ‘time’ 
to do that yet for automated tasks.