When you want to flip the axis of input, like WASD keys.
The first person example swizzles the WS (up/down) axes, but not the SD (left/right) axes, for example.
In the example, the input is essentially passed to the Move Action as a 2DVector like (x, y). If you were to ignore swizzle, you’d be sending the value of WS as the first/x axis, like (WS, 0), which would be problematic when you have to deal with AD’s input in the form of (AD, 0), too.
With it, you’re sending the values of the y-axis (WS) as (0, y), which, together with the values of the x-axis (AD), becomes (x, y).