A princess wears all sorts of clothes, for all sorts of occasions. She may need an outfit for an adventure through the woods, or something for riding a horse, or even an outfit to wear while fighting off pirates! However, every staple outfit of a princess is a dress.
A dress to wear while picking flowers and having a picnic in the meadow with all her animal and fairy friends.
Or a dress to wear during tea time with the Queen-what a delightful way to spend the afternoon!
And of course, if she is any kind of princess, she will definitely need a dress for all the balls she attends every night. It’s important to have a dress as ruffled and as full of a skirt as Belle did. She must look fabulous and breath taking! 
As you can see, for a princess she needs all sorts of outfits, and many dresses to look proper. No wonder she is always delighted when she receives a new one!