Description
Magento provides layout update handles specific for each product e.g. PRODUCT_123
So for each product in each store view there is one layout cache record (with xml layout) even if you;re not using this feature.
if you have e.g. 1000 simple products, than you will also have 1000 cache entries, each for one product. If you are not using layout handles for specific products, than all these 1000 records are the same.
Actually most of the cache entries in the cache backend are cached layouts.
If there were configuration option for disabling usage of these update handles, than Magento would need to generate this layout xml only once. This way layouts could be generated once and reused between products, which would save a lot of rendering time.
The same problem applies for categories. Each category has also dedicated layout update handle which caused layout to be cached separately.