I think this is a very academic sub specialist rad onc opinion...
I can't tell you how much my general internal medicine training helps especially as a rad onc generalist with indigent patients that have little (or zero) primary care. You can't "stay in your lane" and avoid PCP-ish things when you (or just you and med onc or you and neurosurg) are the only doctors the patients have. So having a little knowledge about adjusting BP meds, monitoring blood sugars on dexamethasone, treating UTI's, etc that is picked up in intern year is very helpful. You also get a feel for what "sick" looks like - like does this patient need to go to the ER or can I perk them up with fluids in office? Nothing but time in the hospital lets you get a feel for sick. Staring at CT's in the reading room won't help.
Sure, you pick up on some of that in your rad onc residency/clinic, but I think that intern year is very helpful.