A Smith Chart is useful:

A Smith Chart is a specialized graphical calculator designed for RF engineering. Its primary power lies in allowing engineers to plot complex impedances and calculate matching networks visually, bypassing tedious complex-number algebra. While it is overwhelmingly used for transmission lines and impedance matching, saying it is 'only' useful for those problems is too restrictive, as it can also be used for stability analysis in amplifiers and oscillator design. Furthermore, it is irrelevant for simple direct current (DC) circuits. Therefore, the most broadly accurate description of its utility is that it drastically simplifies mathematical operations involving complex impedances.