Ever hear about regular glass in cabinets? My friend used it for his medicine cabinet and after a hot shower, the temperature shift made it crack. It didn't fall out all at once, but big shards hung in the frame. Took him forever to get it all out safely.
Regular glass cracks when it gets too hot or cold too quickly. Tempered glass is made to handle those changes, and if it breaks, it turns into harmless little bits.