use of trenchblocks

[vonspritz]

Hi all
I'm a bit baffled about the use of trenchblocks (laid flat on the footings) versus two skins filled with a weak mix concrete. Most illustrations show two skins, why not just take the trenchblocks to one block below ground level?
Hope this question is clear as it's hard to explain without a diagram.
Thanks
Vonspritz

[welsh brickie]

Trench blocks save time on labour costs and is a lot quicker.You dont have to place weak concrete in the cavity,you stop the trench block just below ground level.