Why mount rocket artillery on a tank chassis instead of a truck?

Why mount a rocket launcher on a Sherman tank rather than on a cheaper truck?

The tracked chassis could follow armoured units over ground that stopped lorries, while scarce trucks remained available for supply work.

Wartime roads and terrain often prevented lorries from reaching the positions where rocket fire was wanted. The British Expeditionary Force used Indian Army mule units when lorries could not fit along local routes, despite the British Army's mechanisation. A tank could take a launcher onto difficult ground, such as a hill without a road, where a Sherman was already wanted for the fighting. Suitable lorries also had competing supply duties, while the Sherman could fire its rockets and remain available as a tank.