Shield Discipline increases the potency of the damage absorption 'shields' cast by the priest, significantly improving and encouraging the use of these signature aspects of the Discipline repertoire. However, it also increases all healing spells.
By increasing the potency of Divine Aegis's protective shields, Shield Discipline potentially additionally improves every healing spell capable of achieving a crit, by granting not only increased healing, but an increased Divine Aegis shield should the heal crit. Along with [Enlightenment], this further emphasises the Discipline priest's synergy with critical strike.