The ability to gather more details than just the score, of how a given
doc matches the current query, has come up a number of times on the
user's lists. (most recently in the thread "Query Match Count" by
Ryan McV on java-user).
EG if you have a simple TermQuery "foo", on each hit you'd like to
know how many times "foo" occurred in that doc; or a BooleanQuery +foo
+bar, being able to separately see the freq of foo and bar for the
Lucene doesn't make this possible today, which is a shame because
Lucene in fact does compute exactly this information; it's just not
accessible from the Collector.