I think one big aspect is the interaction with other developers and sharing in their way of doing things. It widens your scope and lets you see the code used for te same function in different ways. Some better than what you may have known. Some worse.
Here are the 11 essential areas of knowledge for a web developer:
1. The languages/frameworks you want to specialize
2. Debugging tools
3. Git
4. Databases
5. IDEs
6. Operating system and basic system administration
7. Web servers
8. Web APIs
9. HTTP protocol
10. DNS management
11. Web services