Searing Pain is a Fire-based direct damage spell for warlocks. It is the main attack spell for warlocks tanking end-game bosses due to the added threat, and for the same reason is not generally used for DPS.
In PvP, players may favor it over [Shadow Bolt] due to its low casting time and thus opportunistic nature against moving targets. It also doesn't leave a telltale trail that gives away the caster's location, and deals its damage instantly, without a missile animation delay.
In PvE, the spell has limited uses due to its extra threat. However, the ability to draw hate from a mob may be useful if you are trying to distract the mob from your pet or another player. Against certain fights (See: Shade of Aran) where DPS and survivability are needed, and there is no set threat table, this spell also works well.