Before making a server, I thought coding was cool, but had never done much beyond running some basic programs. Making a server gave me clear goals that helped me learn coding much faster. "I need to figure out how to add custom bosses so my server will be more fun!" is much more fun and interesting to a teenager than "I need to figure out how to remove a Car object from this list of Car objects". There's also the very important social aspect - seeing other real people enjoy code you wrote is extremely satisfying.
And you can do really cool stuff with servers that teaches you real things, moreso than any class could. My own experience writing a ton of custom code for my Minecraft server taught me things like Unix, Redis, MySQL, web dev, reading obfuscated code, networking (like running 8 servers and writing code to pass players from one to another), deployment, basic security, pathfinding algorithms, writing scripting engines, etc. etc. all of which I had no experience with before, and would not have learned for several years otherwise. I'm graduating with my master's in CS now and still haven't been taught in class many of the things I had to figure out to make what I wanted to make. The few other devs of big servers that I knew had similar experiences - started off as beginner programmers, and gained a huge amount of experience from building out a passion project.