If you were unlucky to be involved in a collision at 50 miles an hour
the bolted metal frame is to stop the tank traveling forward.
A 600 litre tank of water hiting you from behind at 50mph well lets say
it wouldnt be nice.
A tank carrying water would also be classed as a live load which means even
if its secure the contents are free to move about inside.
This could cause whats known as a pendulum effect causing your van to tip over.
That why its important to have a baffled tank.