I often get home to find missed calls on my landline with no message. Without a message, all I have to go on is caller ID (1471 in the UK, *69 in the US).
When someone calls me on my iPhone, I get to see the name and know who called, but when it's the landline....
It'd be great if I could look up these phone numbers on my iPhone to see if it's someone I know and should call back, or someone I don't who can just call again if it's important. Unfortunately the Contacts app only allows searching by name.
With WhoRang you can type a few digits of the phone number and see who it was that called. A quick tap (on an iPhone) and you're calling them back.