This is not an intentional indication of activity, but rather just of power.

They don't. The LEDs fluctuations are a by-product of power rail fluctuation and this in turn can be detected to discover some elements of the computations involved.

It's pretty subtle. This isn't like a flashing harddrive LED, it is an LED that to all intents and purposes is burning steadily. Until you look at it in more detail and you realize that it isn't quite as steady as it should be.