LEDs are everywhere and in every project you see anywhere. They show that your microprocessor unit is alive and running the program you wrote. LEDs are tough little lights that light up everything including you. They provide instant gratification. Here is my simple tester. There are lots of kits to show you electronics and soldering and a lot of them use LEDs. Even high-end electronics use LEDs that nobody ever sees to tell a technician that a circuit is working.