The understanding and application of electrode potentials allows us to do one of two things: Build cells or predict the likelihood of a redox equation.
In building cells two couples are required with different values of redox potential. One couple will become the negative pole of the cell, and the other the positive. When we start to put values on the redox potentials it will not come as a shock to learn that the more negative couple will form the negative pole of the cell.
The resulting reaction may then be used to construct a current-delivery device called a CELL although the result may not be of much practical use because the cell reaction rate is too slow. It can't keep up with the demand for current, in other words. And there in lies the drawback of these data. They cannot be used to predict the RATE of the reaction. All they will do for you is to tell you whether it is spontaneous or not.
Spontaneous in chemistry does not always mean fast... all it means is that when mixed together a certain collection of reagents with speciifed concentrations are not at equlibrium, and will move forwards until that condition is reacahed. The reaction will then stop..