Ohms Law:
Voltage = Current(I) Times Resistance(R).
V= IR 
I = V/R
R = V/I

It is, what it is and it is used constantly and continuously by engineers to convert a Voltage to a Current, Current to a voltage, or set a current limit based on a voltage. 

I didn't understand the waterfall analogy and it still doesn't really work for me.  It wasn't until I worked with a power supply, multi-meter and and a resistor at my first co-op that the relationship finally clicked. 

My goal is to provide explanations to a Middle School version of myself how all of this stuff relates and the practical sides of everything. Wikipedia is a spectacular reference for everything that will be covered.

Power, Area and Speed are principle considerations for what's "Practical".

Power determines how much work the system can do and is an important consideration for battery life and how to size components
Area: how small does the circuit need to be, how much power will the circuit need to handle.
Speed: How fast does the circuit need to be to achieve the performance goals. You actually want Fast enough, not fast as possible.