It doesn't.

The date is for the plastic bottles they come in. Chemicals from the plastic will eventually leech into the water over longer periods of time. As far as I know, this isn't dangerous, but it can 'taint' the water affecting the taste, which is why it's a best before date.

Worth noting a 'best before' date is just a recommendation - the product is still safe to consume after this date, it's just that it's likely past its optimum condition.

However, products with 'use by' dates should not generally be consumed past this date as they can start to be dangerous for consumption - ie, fresh non-pasteurised foods can grow harmful mould/bacteria.