The HST provides additional safety. By testing the already tempered glass we prevent the risk of breakage when already installed.
Toughened glass is heated up to approximately 290 ⁰C for a period of time specified in the applicable standard. Close to 99% glass panes with inclusions will be identified and suffer fracture.
There is a growing tendency to require glass used for large projects be HST tested. In case of a fracture the glass panes must be promptly replaced, a very costly procedure often calling for a lifting crane or other specialized equipment.