The reason why bottled water has an expiration date is primarily because the plastic can degrade and therefore cause many core problems. The primary problem that happens with this is that the plastic that is being used for the storage of water may or may not have an expiration date for a longer period, because if the plastic that is being used within the water bottle is not of good quality and has a shorter expiration date, then that water bottle will degrade and cause substances that are within the plastic to leach into the water. And when this happens, the primary reason the water becomes toxic for us is because of the compounds that are present in the water bottle plastic. These are chemicals such as heavy metals antimony and Bisphenol A.
Both compounds are very dangerous for human health and, therefore, can cause severe types of cancer such as skin cancers etc. Not just that, when these chemicals are constantly being ingested within the body, they can also cause several other complications, such as damaging the gut or the elementary canal.
They can also reduce your immunity, therefore making you more susceptible to diseases which could have been defended against, and finally, they may also cause respiration problems.
Bottled water is often carbonated and, therefore, with the progression of time, it may become decarbonated. As a result, it may develop an after taste and the carbon that is decarbonated within the bottle when ingested can cause respiratory problems such as lung cancer and many more.
As a result of this, keeping all these hazards and complications in mind is the reason why expiration dates are mentioned on the water, so that people make sure that this water has not expired and may not cause harmful effects on their health.