There are several good ways to make use of the raven, eventhough it's in every case just a supporting unit.
The easiest one is raven harrass, simply move into the back of the enemy base and put a few auto turrets in his worker line or fire a seeker missile.
For bigger battles, the point defense drone is very effective against most ranged units, allowing your units to survive longer and even overcome an enemy with a slight advantage. It won't turn a battle on it's own though. The seeker missile is imho not that effective in a big battle since the targets it would be very effective against (zerglings, marines) die very fast anyways.
The biggest use for the raven is against banshees, dark templars or to find burrowed units or as spotter for siege tanks (though it's too vulnerable and expensive for that usually).