A cross between a personal webpage, blog, and second brain.
The purported purpose of a digital garden, over a blog, is that ideas aren't meant to be fixed in time. Things change: the state of the world, my knowledge, my opinions. Therefore, instead of formulating thoughts as posts--a chronologically sorted collection of permanent footprints on the internet--it makes more sense to think of them as "plants" to be "tended to".
The real reason though (for me) is to get myself to actually write, since it feels much lower stakes knowing that I can go back and edit things later--that everything is in a perpetual beta.