Gives an indication of reading/writing activity on the hard disk. Allows for an extra indicator to show that the system is working on something.
The iMessage progress bar is a lying piece of garbage. If you are connected to a network but do not have an internet connection it will creep up to 80% and stop despite no progress made sending.
Airpods have a light that can shine white, green, or amber. White is a good indicator that they are in pairing mode. I still have no idea what the exact conditions for amber and green are though. Apparently it flashes amber when the case battery is getting lower, but also if the airpods themselves are not fully charged.
Even better, there is also an error case that uses the AMBER light that pulses. Awful indicator of what the status of the case is.