The superposition principle is used to solve circuits because it simplifies the analysis of complex systems with multiple sources. It allows us to consider each source individually while assuming all other sources are turned off. This simplification is possible due to the linearity of the circuit components. By solving the circuit for each source separately, we can determine the contribution of each source to the overall response (e.g., voltage or current). Then, by superimposing these individual responses, we obtain the total response of the circuit. The superposition principle helps break down a complex problem into more manageable parts, making circuit analysis easier and more systematic.