Here's a use case not well addressed in other sites -

"I live in Vancouver, Canada. I can fly out of YVR, BLI or SEA. I have $800 to spend. Which are all of the destinations I can fly to for $800 CAD return, all-in, in the middle of November?"

Travelocity used to have a feature like this called 'dream maps,' but they dropped it - Presumably due to the huge computing requirements.