For things like bottled water or canned or pickled food the "best before" date is more like a "at this point the package starts to degrade".

For example pickled cucumber that are sold in jars start to degrade faster as soon as the jar lid starts to rust or the rubber coating looses its silkiness.

Or canned food: at some time, the metal starts to react with acidic ingredients and dissolve into the food and spoils it.