Primers ,when not loaded into ammo,are open to atmosphere and moisture degradation. Powder and primer material are actually 'burning' (oxidation) from the time they are made and avoiding sunlight,moisture,therma changes...will slow this process to a point that storage beyond 25 years is possible. In loaded ammo,these components are (hopefully) sealed from the rigors of heat,light,moisture,and hopefully thermal extremes to the point of nearly indefinite storage. I have fired a major quantity of surplus ammo for .308 ,30-06', 38 Special and 45 ACP that were date coded for the wartime effort in 1943 - 1945 and had nearly 100% successful use of such old ammo.
A foam cooler helps keep the away from light,moisture,and thermal extremes...promotes good storage practice and slows the aging of the primers in this specific case.