Thanks to Hello, you can see who's calling you even if you don't have the number saved on your phone. The reason: if the phone number is listed within the Facebook database (which is very probable), the app will show you all of the relevant information when you receive a phone call.
If some of the contacts in your phone don't have a phone number listed for any reason, Hello will give you the option to sync the data manually. This process is simple, fast, and convenient.
Hello is a good addition to the phone app that comes installed on your phone by default, thanks to the fact that you can always (or almost always) know exactly who's calling you.