In case of sheathing with straight-edge panels, an expansion gap of 2-3mm must always be left between the adjacent boards (on all 4 sides) when fixing them to the supports.
If working with T&G milled-edge boards (asymmetrical 2 or 4 sides), this issue is no longer a problem. The T&G profile already has a 1mm dilatation gap incorporated, so this saves time as you can skip checking the correct alignment.
Secondly, when using straight-edge boards for floor decking or wall sheathing, the panels must be supported on all 4 edges, in order to reduce deflection and improve the stiffness. This means the long edge of the boards must be fixed on additional supports (noggins).
In case of roof sheathing, the noggins must be replaced by H-clips to reduce deflection between adjacent panels, and additional bracing stripes must be provided for stiffening the structure. This results in additional labor and material costs, which can be eliminated by using T&G boards instead, which require only their short edges to lay on supports.