You have to think of it as “back of napkin”accounting, which is nice to have on any trade or contract spanning the entirety of the player population. As long as you know the assumptions going in and treat it as a quick and dirty estimate, the tool is awesome.
For example, Hader @ $15m = ~2.5 WAR / season * 1.5 seasons * $10m / WAR - ~$21m in salary. All that seems more or less reasonable, even if you want to quibble with the win projection, market rate for wins, or the details of his arbitration case.