I will make some explaining graphics soon so it is clear exacly what this does. But in short, you do not need the same FOV across games, in fact one of the key points of desktop mode is that you can play with different FOV's, but keep the same physical distance on the screen.
Whether it helps aiming or not is personal preference. The normal calculator uses a mouse-constant distance to rotate 360 degrees. To some people this is the most important, so they know by muscle memory how far to move the mouse to spin say 90 degrees.
Desktop mode on the other hand uses a constant screen distance instead. What this means is that if you see an object on your screen, and want to move your crosshair to that object, the distance you have to move your mouse is exactly the same regardless of FOV. So say an object pops up in game in your lower left corner where your start button is in Windows. The distance needed to move your crosshair here is the same as moving your mouse pointer from the middle of the screen to the start button. Now if you change FOV, this distance will change, hence the different 360 distance in CS:GO and BF. Some games do adjust sensitivity with FOV however.
Some pictures and arrows will make this a little easier to understand!