Why was Hestia worshipped?

Well, Apollo and Poseidon both sought to marry her, but she rejected both of them. After the rejection, she approached Zeus and asked him to protect her. Once that happened, she vowed to never get married and to remain a virgin forever. Zeus felt so honored that Hestia informed him of the rejections that he granted her special blessings. And that is why Hestia is worshipped. It's also because of her hearth.