Why are there sometimes empty (or water-filled) tankers or other wagons at the end and beginning of rakes carrying petroleum products or other inflammable substances?

These empty or water-filled tankers or other wagons are known as 'guard wagons' and are intended to provide a safety buffer for the tankers carrying inflammable cargo. They are intended to take the brunt of any minor collision so that the tankers carrying the inflammable substances are not themselves damaged leading to possible explosions or major fires. At the head of the rake, next to the loco, another reason for providing guard wagons is to prevent inflammable vapours from the tankers from catching fire either from the hot diesel exhaust from the loco, or sparks at the pantograph from electric locos.

Source - IFRCA.org