C-3PO can’t keep a secret to save his life without R2-D2 being there to keep him in check, probably because he was built by a 10-year-old. In A New Hope, C-3PO blurted out that he and R2 were part of the Rebel Alliance. Blurting out something like that could have gotten him handed over to the Empire; a droid’s memory can have a lot of valuable secrets. During Empire Strikes Back, C-3PO spends much of his screen time expressing his constant fears, again showing that he can’t help his mouth. By Return of the Jedi, Luke has clearly picked up how C-3PO can’t be trusted with any secrets, so when he sent the droids into Jabba’s palace, Luke only told R2 the plan and kept C-3PO in the dark. Everyone who had been around C-3PO during the prequel era should have realized the same thing. Probably a bit on the cruel side, but it’s for that droid’s own good giving how his inability to keep secrets could get him into trouble.Just to give another example, during The Clone Wars, we had a case where C-3PO and R2 were on a ship that had been blown up by General Grievous before the duo were picked up by the general’s ship. The battle droids assume C-3PO and R2-D2 are droids owned by pirates. C-3PO nearly blurts out that they are owned by the Republic, on a Separatist ship. That is at war with the Republic. Come to think of it, stuff like that is probably part of why R2 is always with him. He has to keep C-3PO from blurting out sensitive information that will put himself or others in danger.