Suppose I want to keyframe 200 birds or something, taking off two at at time about every second with a bit of randomness. Oh, I want there to be some variation in their colors. And I just realized I want the interpolation on all of them to be linear, not bezier. That's 50+hours of tedious gui work reduced to 4 hours with the API.

Less useful since Geometry Nodes were included, but generative/procedural stuff was only possible via the API for a long time. Besides that, every single addon is leveraging the Python API, so basically any functionality you get from installing an addon.

Besides that, I've written bunch of macros as basically python files I execute in Blender to do repetitive tasks. Another thing I've done is writing a distributed render queue for headless rendering, using the Python API.

I've been using it to automate behavior of an asset pipeline for a game. Basic stuff, like taking many assets and putting them in the right location, combining several things, rendering, on demand (not at runtime, but based on usage in the game), etc.

Neat thing too is it's in Rust, not Python.. because, well, i prefer and know Rust, and by doing it in Rust it works with the rest of my game code so that on-demand stuff i mention plugs in nicely to my existing ecosystem. Note that i'm using Py03 for Rust, so it's just Rust<->Python. Not native Rust support or anything

Basically automating variant creation and modifying assets for various level of details.

Also automatic fixing of imported models from various game ripping sites.