Cinderella

Cinderella is the beloved child of a widowed aristocrat; a kind and devoted father who feels as though his daughter needs a mother's care. He remarries Lady Tremaine, a widow with two daughters of her own, Drizella and Anastasia. After Cinderella's father dies unexpectedly, Lady Tremaine is revealed to be a cruel and selfish woman, only interested in her daughters. Cinderella is mistreated by her stepfamily, who take over the estate and ultimately reduce her to being a scullery maid in her own home. Despite this, Cinderella grows into a kind and gentle young woman, befriending the mice and birds who live around the chateau.
One day, at the royal palace, the King discusses with the Grand Duke his desire for his son the Prince to settle down and have children. They organize a ball in an effort to find a suitable wife for the Prince. Cinderella asks her stepmother if she can attend, as the invitation says "every eligible maiden" is to attend. Lady Tremaine agrees, provided if Cinderella finishes her chores and find a nice dress to wear. However, the extra chores are preventing Cinderella from designing her dress on time. Cinderella's animal friends, led by Jaq, Gus and the other mice, fix up a gown that belonged to Cinderella's mother using beads and a sash thrown out by Drizella and Anastasia, respectively. When Cinderella comes down wearing her new dress, Lady Tremaine compliments the gown, pointing out the beads and the sash. Angered by the apparent theft of their discarded items, the two stepsisters tear the dress to shreds.
Just as Cinderella is about to give up hope, her Fairy Godmother appears and turn the remains of Cinderella's dress with her magic wand into a white ball gown with glass slippers. She also transforms a pumpkin into a carriage, the mice into horses, her horse Major into a coachman, and her dog Bruno into a footman. The Fairy Godmother warns her the spell will break at the stroke of midnight. At the ball, the Prince rejects every girl until he sees Cinderella. The two fall in love and dance alone throughout the castle grounds until the clock starts to chime midnight. Cinderella flees to her coach and away from the castle, losing one of her glass slippers in the process. After her gown turns back into rags, the mice point out that the other slipper is still on her foot. The Grand Duke informs the King of Cinderella's escape and presents the slipper as their only piece of evidence. The King orders the Duke to visit every maiden in the kingdom to find Cinderella, issuing a royal proclamation that she will then marry the Prince.
When news reaches Cinderella's household, her stepfamily prepare for the Duke's arrival. Overhearing this, Cinderella dreamingly hums the waltz played at the ball. Realizing Cinderella was the girl who danced with the Prince, Lady Tremaine locks her in the attic. When the Duke arrives, Jaq and Gus steal the key to Cinderella's room, but Lady Tremaine's cat Lucifer ambushes them before they can free her. With the help of the other animals and Bruno, they chase him out of the house and Cinderella is freed. As the Duke prepares to leave after the stepsisters have tried to force their enormous feet into the slipper, Cinderella appears and request to try it on. Knowing the slipper will fit, Lady Tremaine trips the footman, causing him to drop the slipper, which shatters on the floor. When the Duke thought its now hopeless, Cinderella then presents the other glass slipper, much to her stepmother's horror. A delighted Duke slides the slipper onto her foot, and it fits perfectly.
Cinderella and the Prince celebrate their wedding and live happily ever after.

Question: Why do they organize a ball?
Answer: In an effort to find a suitable wife for the Prince.