FlatGeobuf's spatial index allows you to fetch the features that intersect a given bounding box, without downloading the entire file. This can be helpful when you have a very large file but are only interested in a small portion of it, and want to keep your page loads small and fast.
For the purposes of the example we've highlighted our bounding box in yellow. Pan the map to move the query's bounding box.
Open your developer console's network pane, and inspect the network traffic. Compared to the example which loads the entire file, you'll see that this example makes more requests for the .fgb file, but they are much smaller because we fetch only the relevant sections of the file.