in order to get to where you don't have very high frequencies, you need analog filtering on the input and the output, and you need adcs and dacs suitable for the application. you might also need attenuation, amplification, impedance transformation, and/or mixing on the input and output. all this stuff is analog and requires this kind of analysis to understand
but whenever you can do stuff in the digital domain, you do, because you can easily reduce the error introduced by digital computation as small as you want, and you can redesign your signal processing after your space probe has passed neptune so a soldering iron would be inconvenient. and you can fit arbitrarily complex filtering and other signal processing into an arbitrarily small package, near enough; what would be another precision capacitor in an analog circuit is just another 16-bit coefficient loaded from your terabyte microsd card in a dsp setup, if you're processing a slow enough signal