Manchester

[Sam Flynn]

how does this show of force protect the public from some unknown imbecile in the middle of a crowd blowing himself up?

Not much, really, but there are other terrorist methods (e.g. firearms, machete attacks, weaponised vehicles) that could be better contained by having armed police.

I agree with you entirely about the bombers, however. Not much an armed policeman or soldier can do against a scumbag in civvies wearing a device that could detonate in a millisecond.