Automation is one of the features that make DAWs such powerful music-making tools, with much more creative potential than basic mixer control. Automation can be added to a number of different track types: audio, MIDI, instrument, group, effect, sampler, and VCA.
Deciding when, where, and how to apply automation isn’t always easy. Also, when working with tracks that already contain automation, it becomes too easy to get muddled over which parameters are being automated. An automation curve is tied to its track and the timeline, not to the parts on the track, and this can create problems.
Read more about this here: https://www.musictech.net/tutorials/how-to-use-automation-in-cubase/