I'm no Scientist of Electrical Elegance or anything, but a fine example for me is the LED... if you don't run a current limiting resistor in front (or behind) an LED, they will pull "infinite current" and pop in a matter of seconds or less. Any device that "pulls current" need some sort of resistive load to limit what happens with the current. E=I*R So if you give it 9 Volts (E) and 1000 Ohms of resistance.. I = E/R.. or .009 amps. 9 miliamps. boom. Conversely, if your R is approximately 0.. any number divided by approximately zero is infinity. Obviously we can't divide by zero, but even a piece of wire has a _little_ resistance, the point being your current will go as high as the power producing device will allow, and things break as crochambeau says
It's the same thing in some power networks on pedals, the 9V in runs right through a 100R resistor to "stabilize" the power source. I had a couple pedals that were noisy as balls, threw a 100R in series with the + wire, way quieter. Theres science and big words, but I don't know them. I tinker