It was to retain both Holtby and Samsonov. That would allow Washington to protect either of those two, knowing whoever is exposed between them is off limits to Seattle. Washington wouldn’t want Samsonov to get selected by Seattle in the expansion draft today, then have Holtby leave in free agency next summer to land the Bobrovsky contract elsewhere if Washington can’t afford to offer him that exact deal.
So at the cost of a second-round pick (originally Colorado’s), the Capitals get to keep both Holtby and Samsonov for 2019-20. Then, if Holtby does sign elsewhere for 2020-21, at least Washington still has Samsonov with another year of experience under his belt to take over the net.
Seattle wouldn’t want Copley, there are much better options available, so he wouldn’t need to be part of that trade equation.