This was a great glyph for Destruction warlocks who specced Conflagrate. With this Glyph applied to their spellbook, the spell no longer consumed Immolate or Shadowflame. The glyph allowed the caster to use Conflagrate twice during the duration of a single Immolate spell and left the DoT to run its course for extra damage.
Warlocks could cast Immolate, follow up with an Incinerate (or Chaos Bolt if specced for it), and then cast Conflagrate as the spell was on its way. The result was that both spells struck at the same time for a massive burst of damage which was usually more than enough to kill non-elite mobs, and even if they don't die, the dots would finish the job shortly.
Patch changes
Patch 5.0.4 (28 August 2012): Changed from Reduces the cooldown of your Conflagrate by 2 sec.
/Patch 4.0.1 (12-Oct-2010): Changed from Your Conflagrate spell no longer consumes your Immolate or Shadowflame spell from the target.