If the iMac works for you, then you don’t need a ROCK. But there are two/three primary advantages.
First, ROCK is ultra-low maintenance. Updates happen out of sight and are usually really quick.
Second, there will likely be times where Roon needs to do some intensive work (database updates, etc) that could conflict with what you are doing on the iMac. But you’re the best judge of whether that’s going to be a problem.
Third, there could be some sound quality advantages with the ROCK. They’d probably be fairly subtle. But one audiophiles ‘subtle’ is another’s ‘life-changing’.
If you aren’t in a rush, why not see how the iMac works and go from there? If it works great, then these are moot points.
Just to be clear, the iMac is wired to the router? Having it work wireless is more of an issue than all three of the other points combined. Roon needs it’s data. It’s network intensive and gets testy if it can’t get what it wants. You can give it a try, but its really not advised.