The thing is . If he can be quick over a whole race / season , why did Renault not give him the chance ?
I think they had a better choice of proven drivers with Sainz/Hulk. Williams seem to be towards the bottom of the barrel on the driver market so probably saw reason to take a punt on him.
I'd be surprised if there weren't certain performance clauses in his contract to insure them against him having issues over the season. Doing weekends consistently over the course of a year would be very different to the odd FP1 or moc-race run in a test session.