Second, outer space is free of seismic noise and most other sources of vibration. The big distances would make it possible to detect gravitational waves with frequencies of .0001 to .1 hertz, much lower than the frequencies for which the ground-based detectors are optimized. I don't think LISA has been funded yet, but if all goes well, it may fly within 10 years or so. You probably heard how COBE, the Cosmic Background Explorer, detected slight anisotropies in the microwave background radiation. They may also help us better understand the X-ray afterglow of gamma ray bursters.
Some folks think they're caused when a supernova creates a black hole.
But nobody is sure. OWL would orbit the earth, watch these things, and figure out where the heck they're coming from. But as usual, the really exciting stuff will be the stuff we can't predict.