Though Sati is an incarnation of Adi Shakti, the reason for her incarnation is not only to meet Lord Shiva, but also to teach True Devotion & Spirituality to the whole world! In the Spiritual sense, she depicts herself as the Soul, since she has taken birth as a human, she behaves like a human.
She portrays the forgetfulness of the soul... how the soul forgets that it is has always been & will always be the part of the Supersoul i.e. God. That's why she is not able to remember who she actually is, Shakti... that we all souls r... we r all part of God, we r Prakriti, His Shakti in essence, but due to the influence of Maya, i.e. Illusion, we get separated from Him, our Real Identity. Although, just like Sati has portrayed, we also do feel a connectivity with the Divine, but that connect is not complete, until & unless we develop the Intensity of Love, Devotion & Surrender that Sati, the Greatest Devotee of Lord Shiva, has for Him! And to develop this degree of Love, Devotion & Surrender, is the responsibility and duty & Sole Purpose of each & every soul in this human birth! That itself is only possible when a human being, thru meditation & devotion, regains the Purity of Soul, that is required to reach Him.
As Swami Vivekanand, the Youth Icon of & one of the Greatest Saints of India, has so clearly stated, the Purpose of Human Life, in just one line: GOD is to be worshipped as the One Beloved, dearer than everything in this and next life.
Lord Shiva, on the other hand, portrays the immense restlessness of God to meet His part, the soul, which has separated from Him, because of the thick veil of Illusion that this materialistic world & sense pleasures have put on all of us humans, since we all r part of Him only, just like every drop of the Ocean is part of the Ocean only! God is always guiding us, showing us the path that takes us towards Him, but we, due to our Ignorance, never pay heed or put an effort to follow that path that takes us to Him!
So, this is the essential massage that has to be delivered to the world, for spiritual upliftment of humanity, because of which God, even though He is One, plays His Divine Play... by separating Himself into two different forms... like Shiv Sati, Shiv Parvati, Radha Krishna... ❤️