Time is the issue
What everyone forgets is ships and aircraft take a long time from procurement to delivery and operational readines. Lets say god forbid WW3 breaks out and we have to fight a real war, Chinese, USA, Russia or the whole 3'rd world. If the UK needed a nuclear deterant at short notice it cannot be built, carriers cannot be built quickly or fast jets at that matter. Frigates give you a small fast platform to load with weapons at short notice, They look quite pointy so would be much faster at reaching trouble spots than a merchantman with a flight deck.
Yes we didn't have the right gear for Afganistan but we could buy it and deploy it within a year or so. We could mobiles and arm the whole country quicker than building even a half dozen ships. My view is large procurements like this is about having the platform available when you need it.