Ref T12539. See PHI190. Currently, each Diff has a revisionID, but Revisions do not point at the current active diff. To find the active diff for a given revision, we need to issue a separate query.
Furthermore, this query is inefficient for bulk loads: if we have a lot of revisions, we end up querying for all diff IDs for all those revisions first, then selecting the largest ones and querying again to get the actual diff objects. This strategy could likely be optimized but the query is a mess in any case.
In several cases, it's useful to have the active diff PHID without needing to do a second query -- sometimes for convenience, and sometimes for performance.
T12539 is an example of such a case: it would be nice to refine the bucketing logic (which only depends on active diff PHIDs), but it feels bad to make the page heavier to do it.
For now, this is unused. I'll start using it to fix the bucketing issue, and then we can expand it gradually to address other performance/convenience issues.