May I know how this projection affects the map? Tried messing around with the inputs but nothing changes and it seems not one of the dependencies as well?

Good question! The projection is not used directly, since the map data is already projected. However, it's given as a reference if you want to overlay more data on the map. For an example, see https://observablehq.com/@d3/u-s-state-capitals

We give this one with pre-projected path data because it's faster to render. I created a fork with a live projection if that works better for you: https://observablehq.com/@observablehq/d3-u-s-map-live-projection

Now I understand better how the `us` data here differs from the `us` data at https://observablehq.com/@observablehq/us-geographic-data.