Building on what Frederf said, the purpose of aerobatics in flight training isn't to teach a maneuver to use in combat (though the barrel roll can be useful in forcing/preventing overshoot in a stern chase), but rather to build confidence in the plane's envelope and build the skills used in air combat (sustained g turns, proper use of the vertical, etc.). Great thing about the cloverleaf is that it trains you in the vertical but also gets you thinking in all three dimensions (four if you include time) simultaneously. Being able to process all 3 axes is a big step in flying, especially tactically.