You don't have to use the same FOV as Overwatch, Viewspeed V2's goal is to eliminate the need for that. However, for a perfect 1:1 ratio in hipfire, you must use the same FOV for it to be accurate. Accurate is probably the wrong word, for the 360 distance and the relative distance your cursor moves on the screen to feel and be the exact same you need to match FOV.
Viewspeed feels the same regardless of FOV, at least when it comes to aiming. Where the argument comes is when you start talking about movement, as being able to control your character with the same 360 distance is valuable for that. (although, in my experience I don't really notice a serious difference in movement capablities) Matching the FOV to Overwatch means that your hipfire sensitivity won't only feel identical, it will be identical in every respect, which is why it's often the recommendation if you can stomach having the same FOV. When you are aiming down the sights or using scopes of any kind is when you are required to use something like Viewspeed, however.
There are some games where it is very undesirable to change the FOV, or some games have it hard locked, this is where Viewspeed comes in as well. Games like Quake which play best at the default high FOV come to mind, or games that were designed with a low FOV in mind, like many console ports.