Quite some people here dismiss requirements engineering as something non-agile and ancient.

If you are in this camp, consider educating yourself a bit on the V-model [1] and notice that this is not only used in the waterfall model, but that it is a way to decompose problems and verify that everything works properly.

This may not be required for a small hobby project, but if you start working at something with multiple companies in various technologies, it soon becomes extremely useful.

[1] https://en.wikipedia.org/wiki/V-model