It's small enough to get a rough feel for how complicated a module would be. It's small enough that it's like "hello world" - test your toolchain (etc) install. It's simple enough to start understanding the process without having to read a bunch of docs. It's straight-forward, so there's no need to both puzzle out the module interface AND someone's module logic.

There are probably plenty more reasons I'm not thinking of ATM.

Ok, then say "Small redis module example to test your toolchain".

Exclaiming it only took 15 lines of code is mostly irrelevant.

"Small" is relative and doesn't tell me anything. "15 lines" tells me this is quickly digestible and I can take a minute or two to look it over.

The idea was to illustrate how little boilerplate is required, that's all.

[EDIT] - and to have the tutorial fit on a printed A4 page.