The Church's goal is "a certain uniformity of posture within the congregation" — because uniformity expresses unity. That said, Rome explicitly clarified in 2003 that posture norms are "not intended to regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free." Personal piety that leads you to kneel at additional moments (such as after Communion when others stand) is not forbidden — but disrupting the assembly's unity by conspicuous difference goes against the spirit of the GIRM.
📖 Congregation for Divine Worship, Prot. n. 855/03/L (2003)