Mainly Album and artist images. The Roon App needs them to function, but there is no need to backup them up to iCloud storage as the app can rebuild what it needs if your phone goes pop.
1.79 GB seems excessive, some users have reported the data space keeps growing, If your concerned about that delete the Roon app from the phone, and then reinstall the app. Then you can monitor over a period of time to see how it trends.