Agreeing with @Hold to the Rod that the teraphim represented the family's ancestors (or local deities) I interpret Rachel's action as a continuation of a basic theme in Genesis: the younger child obtaining the blessing of the ancestors by whatever means.
Jacob, the younger son, obtained the blessing of his father and his ancestors through deception. (Gen 27)
Rachel, the younger daughter, deceived her father in a similar manner, in order to receive the blessing of her ancestors.
In the end, Laban gave Rachel's family his blessing, just as Isaac did for Jacob.
Genesis 28
[Isaac said] "Go now to Paddan-aram, to the home of your mother’s father Bethuel, and there choose a wife for yourself from among the daughters of Laban... 4 May God extend to you and your descendants the blessing of Abraham, so that you may gain possession of the land where you are residing, which he assigned to Abraham.”
Genesis 32
Laban kissed his grandchildren and his daughters and blessed them; then he set out on his journey back home. 2 Meanwhile Jacob continued on his own way, and God’s angels encountered him.
The parallel between Rachel's course and Jacob's is clear: each of them deceived their father in order to obtainand thereby obtained an ancestral blessing. Thus, the two lineages of Isaac and Laban would merge in Jacob's family under the ultimate blessing of God.
Conclusion: Rachel stole her father's idols in order to obtain the blessing of her ancestors, just as Jacob deceived Isaac in order to receive the blessing of the patriarchs before him. In the end both ancestral lineages also received the blessing of God. The unexpected blessing of the younger child is a frequent theme in Genesis.