If you have a suspicion there's a hidden or invisible unit in an area, you can Flare to reveal any of them. This can be especially useful in PvP or in places where you know there are stealthed mobs, such as the western wing of Dire Maul.
Can be useful for finding invisible creatures, such as the elite hunter pets in Pandaria, eg. Patrannache.
Notes
Using Flare leaves a bright light on the ground, possibly alerting stealthed units you are nearby.
There is a brief period of time after using the spell, that a rogue, if properly positioned, can sneak in before the flare effect begins and get an opener off.
Trivia
Flare is a rare Hunter spell in Hearthstone. The flavor text reads: "Not only does it reveal your enemies, but its also great for parties!"
Patch 4.0.1 (2010-10-12): Now has a 40 yards range, up from 30 yards. 
