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

When do you need it?

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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.

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