That used to be one of those 'Bloggs' questions we used to get at CFS groundschool during teaching practice sessions:
"Sir, why do aircraft like the F-15 have 2 fins?"
"To improve directional stability at high Mach No., Bloggs"
Followed by a 4-colour diagram of CL alpha curves, yaw angle etc.
This was fine, until one day I was playing 'Bloggs' and asked my colleague:
"Sir, why do aircraft like the Shackleton have 2 fins?"
"To...err, ahhrrghh - you bugger!"