I never use phone contacts because I remember all the phone numbers I could possibly need (partly because almost nobody uses phone nowadays so I don't have to remember a lot of them). I never accept calls unless I know the number or expect a call either, but I don't need contacts for that.

The article clearly explains that using an external memory device allows users to offload low importance information and focus on remembering high importance information:

This was because using the device shifted the way that people used their memory to store high-importance versus low-importance information. When people had to remember by themselves, they used their memory capacity to remember the most important information. But when they could use the device, they saved high-importance information into the device and used their own memory for less important information instead.