Contacts Questions

If you tap on the blue 'i' in a circle in the right you should be able to see which number was used. Also if you label numbers within a contact (like home, mobile, work, etc.) then that label will show up letting you know what number was being used.

 

If you click on the (i) on the right it should show you what number that call was made from.
The contacts app and the phone contacts bring you to the same content.
Its an additional way of getting to the same place I guess.

 

The code and data to support contacts functionality (like its presence in Phone and Contacts) remains, even if you remove the app's icon from the Home screen. You can de-clutter your Home screen a bit, but you won't free-up storage space.

Support for contact info is a core necessity for email, text messages, calendars (information about the persons/locations in the appointment, birthdays), point-to-point driving instructions... It's hardly just for phone numbers. It's not just there to support Apple's built-in apps, but any app that needs to access the kind of info stored in contacts - a single address/phone book available to any app on the phone. For that matter, "Hey Siri, call Jane!"

The presence of a separate Contacts icon (whether using the app provided by Apple, or using a different contacts app) also gives you a more direct way to add/update/access contact info and to initiate an action.

Open Phone > tap Contacts > find contact > tap phone number
Open Contacts > find contact > tap phone number

Open Phone > tap Contacts > create new contact
Open Contacts > create new contact