Imagine I call the number 123-456-789 one day, but I don't have it as a contact. Three days later 123-456-789 calls me back and I don't recognize the number, so I don't pick the call. I want to be able to know that I have called 123-456-789 without having to either memorize it or save it to my contacts.
When you interact with clients and prospects multiple times for some days or weeks and later forget about them it is somewhat unpractical to add everybody to the contacts list.
The same app could also fetch information from the Messaging/SMS database and show something like:
123-456-789 is calling you, you have called it twice in the last week and it has called you once, a call that you've rejected. Also, there's an SMS message from it 2 days ago.
(Of course it doesn't have to be a free text like that.)
If possible, the app could show information from other sources too.