Safe phone calls
When the President makes a phone call, they don’t just use any old phone, they need to use a secure line. They never can just make a phone call, even if it has nothing to do with national security. Everything needs to be checked so spying can not occur or information can not be stolen.
Who knows, maybe if the president has an argument with his wife over the phone and another spy agency hears it, they now know something about his personal life. We don’t really know what they could do with that information, but it’s better if they just didn’t have it at all.