The reason for cutting off most of the branches is that it allows a strong root system to develop, as roots will form all along the buried stem.
The reason for digging a long, shallow hole, and laying the seedling sideways is that the soil is warmer at the surface.
It might seem counter-intuitive to cut off most of the branches of the seedlings you have carefully raised and lovingly tended, but it really helps you grow a stronger tomato plant.
My tomato plants routinely grow to be 8 – 9 feet tall.