Firstly, it acknowledges the idea that many people have someone exceptionally significant in their life, and for the speaker, that person is the one being addressed. This suggests that the individual holds an irreplaceable role in the speaker's heart.
Secondly, it emphasizes the exclusivity and specialness of the connection between the speaker and the person they are referring to. The term "special person" implies a bond that goes beyond ordinary relationships.
In essence, this statement is a declaration of love and appreciation, affirming that the person being spoken to is the one who holds a profoundly unique and valued position in the speaker's life.