The play itself doesn't have Lord Capulet giving any reason except that he tells Paris "tonight I hold an old accustomed feast." It seems like a party just for party's sake.
The event does serve several purposes though. Capulet further explains to Paris who is wanting to marry Juliet that this would give Paris an opportunity to woo her or flirt with her as well as compare her to Verona's other beauties making sure that he indeed likes Juliet above others.
During the party, it also serves the purpose of letting Romeo and Juliet meet and building Tybalt's rage over Romeo's presence. These are very specific inciting incidents that much action later on in the story hinges on.