I have still not seen 1 shred of evidence F3 mode helps shooting in any way.
I have seen many claims of other people using it, but never one person saying exactly how he uses it himself to shoot with.
HiTech
I can attest that it does help. Think of it as flying around zoomed out with your interior cockpit turned off. You can pull lead on planes totally outside of your scope if you were inside the cockpit. Because you are in trail position with the "view camera" you can easily bank into a target cutting across you at 90 degrees, aim, and precisely hit them with a stream of ammo before anybody inside the cockpit can spot them.
In HTH rooms way back it was the main way most folks flew. It's very easy. Coupled with increased ammo (in HTH, not in the DA) it was quake-like easy kill mode.
In the MAs it's used by boston and IL2 pilots (and the odd B-25 pilot that wants to dogfight with the guns package) for easy aiming and shooting. In a Ju87 it makes it easy as pie to follow a target trying to get out of your guns. Your plane's performance holds you back in these examples, but often you get a big advantage from F3.