When do GeoJSON feature collections need feature ids?

Question

I've seen feature collections with feature ids for each feature. I've seen others with no featureid.

When do you need it?

Answer 1

Typically any system that has/transmits some unique information identifying a record sends it so that modifications to that record (in this case, a feature) are recognized in later analysis.

That is, if a piece of software sends out subsets of itself as GeoJSON, or accepts GeoJSON as input to modify an existing data store (say a table in PostGIS), then a unique identifier is needed. However, in general, GeoJSON does not require any unique identifier for simple data interchange and no such concept is expressed in the standard.

In short: in GeoJSON, there is not any defined situation where it is required. Downstream applications may require it to match up GeoJSON records to records in a database.