The only reason Lord Capulet offers for hosting the feast is that it is one he normally celebrates. When he invites Paris to attend in Act I, scene ii, he calls it "an old accustom'd feast."
This traditional party, however, turns out to be very significant to the plot of the play. Capulet invites Paris to come because Paris wants to marry Juliet, but Capulet does not want his daughter to marry so young. He tells Paris to come look over all the ladies in attendance and see which he prefers. He agrees to allow Paris to court Juliet and to abide by Juliet's wishes. It is this feast, however, at which Romeo slips in uninvited, and he and Juliet meet and fall in love.