(E) is a POTENTIAL which is measured in volts
(I) is measured in Amps
, and is what happens when electrons at a given voltage are permitted to flow through a circuit (i.e., the circuit is completed).
(E), therefore, is a measure of how much current
(I) CAN flow across a given resistance
(R) if permitted to do so.
Voltage (that is, the potential) exists, whether electrons are flowing or not. Measuring a given potential, however, requires that some (usually tiny) amount of current be permitted to flow through the measuring device..