Music automation is a precious tool. A modern DAW project can include dozens (or even hundreds) of parts. There is no way you can stay on them all with a single pair of hands and eyes. By automating the settings, you can add sophisticated details to your projects which will be perfectly reproduced each time you play it.
The simplest type of automation is probably a fade of volume. By automating the volume of a room in your DAW, you can bring out a new part of nothing, or create a fade at the end of a track. But automation in music goes much further than that.
Most modern DAWs (such as Logic Pro, Pro Tools and Ableton Live) allow you to automate parameters through a project, synths and effects with global parameters. You can use automation to create filter sweeping, give a complex movement to a synth, activate and deactivate the effects, or even change the tempo of your song halfway.