There are two reasons. Evolutionarily speaking, redundancy is often a huge benefit, especially for crucial organs. The auditory system is the primary sense for felines, so one cannot underestimate the benefit of redundant ears. But the reason for the existence of every organ is multi-faceted, and the ears are no exception. [Humans are able to sense the direction of sounds as the product of having two ears](https://ehomerecordingstudio.com/binaural-hearing/) because of the delay of time between when a sound can reach one ear to the time a sound can reach the other ear. This becomes more difficult, though, when the sound comes from a place like directly behind the head. A sound coming from directly behind the head reaches both ears at the same time, making it impossible for the human to sense which direction the sound came from. For all the human knows, the sound came from directly above, or directly below. Catgirls solve this problem by having quadaural hearing, or hearing with *four* points of reference, allowing them to sense the precise location of nearby headpats without the need to open their eyes.

TL;DR: She has two sets of ears for redundancy and improved sound location.