Tier point runs (specify one or both legs in business).

Qualifying flight runs, e.g., if you have 560 TP from Qatar and need 4 BA flights for silver.

Or just because you want to get away for the weekend but don't have anywhere particular in mind. I'm not suggesting that I'd get on a plane to somewhere completely unappealing just for the sake of flying, but I would never have thought of Quimper as somewhere to go at the drop of the hat, but it turns out it's really cheap this weekend.