"other than to clear Liddy as the 'leaker'?"
Well it may clear Rove too if Rove said he heard (or may have heard) it from Libby. Fitzgerald may have determined that the Intelligence Identiites Act was not violated a while ago. All he was left with was obstruction of justice if any of the witnessess lied. If Miller's story syncs up with what Libby has been saying and in turn with what Rove has been saying, that would clear the obstruction charges.
But if she told Libby, it seems strange why she needed a release from him. Unless she was not sure what Libby told Fitzgerald (and didn't want to get a source of hers in trouble if the stories differed).
Who knows?!?!?