I suggest every developer write a database from scratch at least once, and use it for something real. Or, even better, let somebody else use it for something real. Then you will know "why database".

My first time was with a Bukkit plugin as a kid. One of my updates broke existing flat json files. Someone asked me if it has MySQL support, I didn't know what that was, then realized oh this is nice.

There are also things besides databases that I'll DIY and then still wonder why so many people use a premade tool for it, like log4j

It's indeed an amazing design and implementation space to explore. If distributed it is nearly comprehensive in scope. (However, did lol @ your "every developer" - that's being super kind and generous or "developer" is doing heavy lifting here.)

Hm, sometimes opening a book could do wonders? But these were the old times...

I'll do one better.

I suggest every developer learn how to replicate, backup and restore the very database they are excited about, from scratch at least once. I propose this will teach them what takes to build a production ready system and gain some appreciation for other ways of managing state.