The possession shows that the noun belongs to proper nouns.
An apostrophe is used to show possession in a sentence.
Look at these examples:
1. (a) The girl took the ball of the boy.
(b) The girl took the boy’s ball.
2. (a) They print and publish the books of the children.
(b) They print and publish children’s books.
3. (a) She is a member of the association of nurses.
(b) She is a member of the nurses’ association.