Firstly, what was once called a 'region' in Pro Tools is now called a 'clip'. It's a block of audio displayed on the screen that can be edited or processed in various ways without affecting any other audio in the session.
In any mix, it is likely that at least some faders will have to move during the mix. A mix that is static all the way through is either very rare, or not yet perfected.
Traditionally this has been done with fader automation, like in physical automated mixing consoles.
This is good, but the problem is that automation was developed in the days when the fundamental unit of audio was the track.
With hard disk recording, the fundamental unit of audio is the clip. There may only be one clip on a track that lasts the full duration of the song. But there will more likely be several clips, perhaps many.
So previously in Pro Tools, although detailed editing and processing of audio was done in clips, automation applied to tracks.
Now, automation can be performed in a much more useful way.
Clip-based automation in Pro Tools allows the level of an entire clip to be adjusted, with a corresponding increase or decrease in the height of the displayed waveform. Or you can set an automation curve so that the clip varies in level to whatever amount of detail you want.
Anything that you could have done with traditional fader automation can be done with clip-based gain, and it is very much easier and more flexible. Any clip-based gain you apply will remain attached to the clip as you move it, copy it or edit it, or even move it to a different track. Fader automation is still there if you want it.
Of course, other DAWs already have this facility. Indeed there are DAWs that have come and gone and turned into fossils that had it.
Even so, it is well worth appreciating the difference between track-based automation and clip-based automation. And using it too!