We live in an analog world. Analog sensors measure such things as temperature, sound and movement. Most of the time, these analog measurements are converted into digital signals for processing. However, when processing has been completed, the digital signals have to be converted back to analog in terms of audio, or to drive actuators.
There are several key differences between analog and digital circuits. First, analog circuits do not scale like digital circuits. Features cannot be shrunk from one process node to the next in accordance with Moore's Law, and there is debate whether they can be developed at all using 10nm processes.
Second, analog circuits immediately respond to their environment in a continuous motion. Digital circuits are discrete and work using a clock to determine when things are recalculated. Unfortunately, clocks create large amounts of noise that can affect sensitive analog components making the design of the two together very difficult.
Third, analog designs have had little success using any form of automation or even effective reuse. It is automation that has enabled digital designs to keep pace with Moore's Law since 1965 and analog block have remained approximately the same size that they were thirty years ago.