The full phrase is for the sake of your prayers
. The words for the sake of
points to why Peter enjoined self-control and sober-mindedness. Clarity of mind is essential not first of all to be able to give a good defence (1 Peter 3:15) but to be able to pray. The disciples had once asked Jesus to teach them to pray and Jesus in reply told them to say, Father, hallowed by your name. Your kingdom come
(Luke 11:2). The return of Jesus Christ as Judge represents the fullest answer to these petitions (particularly the second); hence the need to continue in prayer. Meanwhile, surviving in a world of hostility requires heavenly protection and strength, and hence also the need for ardent prayer. In fact, the primary resource the Lord has given to Christians to survive in a hostile world is prayer (Luke 21:36; Luke 18:1; Ephesians 6:18).
7 The end of all things is at hand; therefore be self-controlled and sober-minded for the sake of your prayers.