Frequently runners keep running in the same shoes for much more than they should- going by the fact that the shoes don’t look “worn out”. I have known runners to run in shoes till the sole peels off! This is a highly unhealthy thing to do.
Running creates a wear pattern on shoes and this if excessive can cause your running form to change and your feet to fall differently. This, in turn, can lead to excessive abnormal forces on joints and muscles, leading to injuries.
Secondly, the foam in shoes dries out in 2 years and loses its structural support even if they have been sitting on the shelf and never been worn. So unless you know the manufacture date you might not know if the foam is still in good condition or not.
This is why you don’t want to buy old models or those at huge discounts. These have probably met their expiry date. To make sure that your shoes are providing you with the support that you need while running, it is best to follow the protocol of changing your shoes every 600 to 800 km or one year whichever comes first.