(I have a Sparkfun RedBoard, but this question seems to apply to R3 Unos and Uno-compatible boards.) As I was building the first circuit in my SIK guidebook (add a resistor, LED, hook it up and make it blink from code), I noticed a blue LED on the board itself did everything that the circuit's LED did - blinking according to the programming.
Why is this LED here? What use cases is it for?
What kind of circuits will I have to adjust to account for this LED?
Is pin 13 traditionally a debug or a troubleshooting pin? Where did this convention come from?