I’ll explain with an example. Suppose you have some undeployed medium machinegun soldiers arranged in a north-south line. You now want to deploy them so that they all face due west (all parallel to each other and perpendicular to the north-south line).
This presents a problem, if you select them all and hit the deploy button then they’ll all face a point located where the cursor is. The axes of their fields of fire will intersect at the point where the deploy order is given instead of being parallel to each other. The desired arrangement can be achieved but it would require selecting individual machineguns and giving each one a separate deploy order.
If you could tell them to deploy to a line rather than a point (or more specifically, to points equally spaced along a line, one point for each unit) then this unnecessary micromanagement would go away. Custom formations already does this for move and fight orders which is why I’m suggesting adding deploy and also turn to it.
It could be argued that the scenario I painted above doesn’t happen all that often in real games which of course makes this a “nice to have but not indispensable” feature. I defer to the devs on whether to include this or not.