When Build & Run (or Cmd+B) is selected Unity opens the project in Xcode and triggers a Clean and then a build. This Clean shouldn't be triggered if Append is selected.

To repro:
1: Open attached project or create new (attached just has bundle identifier set)
2: Build & Run for iOS, select Append. (Append should also be selected automatically with Cmd+B)
3: Once the loading in Unity finishes see that a Clean is triggered in Xcode (probably says Clean Successful in Xcode, might get a notification too)