The demonstrations of security are just that:

a) to make the insecure feel secure
b) to create an image of "we are under attack and on the attack, big and small, and we are doing much to win a 'war' "
c) to catch a lone-wolf idiot with ill intent and absent-minded fools
d) to deter and punish those listed in Item C
e) to push larger-scale terrorist plots away from inflight scenarios

It appears to me, that the emphasis is approximately as follows:

50% a; 20% b; 15% c; 10% d (especially for unpopular ethnic or religious minorities); 5% e.