It's a simple traditional greeting to acknowledge the call is received. Total silence after answering would be sort of strange.
Having said that in this day of cell phones many people just answer stating; This is (whatever answerer's first name is).
Sometimes just saying hello can maintain one's anonymity.
In many instances they allow unknown callers to go to voicemail.
If it's a known caller they greet the caller by (their name).
Thank God for caller ID! (It was a "game changer")