It is my view view that the Pinan / Heian series records a complete combative system. It is also my view that the kata present the techniques in the order they should be learnt and studied.
Among other things, Pinan Shodan (Heian Nidan) teach us how to control the enemy's limbs. Pinan Nidan (Heian Shodan) builds on that and teaches how to both lock the enemy’s limbs and escape from attempts to manipulate our own limbs. Having learnt those skills, the sequence shown in this clip (from Pinan Sandan) gives us an alternate counter should the previous ones fail.