ROM Manager is an app that was designed to work alongside ClockworkMod Recovery. Both are made by the same person and one is an extension of the other. In a strange twist of events, Developer Koush has added support for another recovery, TWRP (Team Win Recovery Project), to ROM Manager. Good Guy Koush, huh?
He also added support for incremental updates, meaning if you have a previous nightly zip, instead of downloading the entire new nightly it’ll only download the difference in files. This will save time and bandwidth, a great idea for those who download nightlies every night (hey, some people get lazy). Both features are in beta, so do use them at your own risk, but they should work fine.