The Jazz made some use of their cap space
The Jazz have plenty of cap room at their disposal. Since they are not really in a position to win at the moment, it would have been no skin off their nose to use it on someone who they never really had any intention to play. Ergo, welcome back, Russ!
This could also mean other trades are coming where Jazz absorb expendable and/or overpaid players into their cap room in trades that could get them more assets in the near future. By acquiring Westbrook, the Jazz acquired $4 million of dead money, but at least there's no financial commitment long-term.
The Jazz managed to use some of available cap room to some benefit, and they maintained long-term flexibility on top of it.