Really just a quick diagnostic LED. Mostly for testing to make sure we are getting a good signal. Since we can't see electrons directly we need to convert them to photons so we know there is a good signal going into the MPU. Plus, and maybe most importantly, they look pretty
Future revisions may leave them off. But if they prove helpful for others to be able to just tell us what LEDs are active we might be able to do Internet diagnostics without having to send the boards back. Something like: one of my injectors isn't firing, your board is bad. We: what do the LEDs show? All but one input LED is blinking. We: you have a break in the harness or connector and the board is not getting the signal to fire the injectors. No need to pull out the meters or teach someone how to test components or measure anything.
Or all LEDs on input blinking but one on the output is not. Then dead board. Send it back. Makes things simpler for the layman