I really love whenever the "theoretical" world of bits and bytes meets the "real" world. When you can see your algorithms control a motor, LEDs or whatever. Also when you get data from sensors then you process information from the "real" world into the "theoretical" world.
However, with embedded systems you already can do a lot. You can display the data can control something based on sensor data and so on. But I think it is a lot more fun when this already complex system is "speaking" with another very complex system. The PC or a Smartphone. This is giving you a lot more possibilities and some targets are easier to reach.
Lets say you want, for any reason, want to publish sensor data to the internet. Of course you could implement an web server on that embedded platform but you also can use a PC to collect the data from the embedded system and publish it using very simple software. The embedded system can be less advanced because you can do calculations etc on the PC.
This of course is meant for private use and playing around with the technologie. In Industry this would be different because you may save money with the smaller embedded system but spend more for the PC/energy costs.