All the time. Say you have a 12V battery and a 3V lightbulb. When the light's instructions ask for a certain number of volts and amps, you've gotta use resistors or anything else you can throw at the problem to get things balanced right. Ohm's Law, Kirchoff's Law, Series and Parallel...they're all just tools for getting the right voltage and right current to the right location.
Hands down, my best recommendation is allaboutcircuits.com. I don't know what your experience level is, but if you can get through Ohm's Law to Series/Parallel circuits...and practice it enough for it be second nature...you'll have no problem with DC systems.