In some scenarios, it is beneficial to avoid the cost of deserializing values when walking a cursor across an object store. IDBIndex has openKeyCursor, and it wouldn't be difficult to support the same method on IDBObjectStore.
Excellent summary of the use cases in this message:
We shouldn't add this to v1, but it's a nice simple addition to v2.
Specified in https://github.com/w3c/IndexedDB/commit/0f168cc96e38441b9131124786992589fbdb0388