Kuldevi/Kuldev is different from Isht Dev. Kuldevi/Kuldev is for entire Kul/Family. Isht Dev is for each individual.
Everyone has their Isht Dev as per their Kundali. And everyone should worship their Isht Devta/Devi daily. Isht Devta literally translates to ‘the favored deity whom we worship, and is the higher power, to whom we connect spiritually. He/She guides us through life, helping us live a fulfilling and protected life, and then finally directing us into leaving the cycle of rebirth behind to experience oneness with the One Supreme Self. Isht Dev gives us intelligence, courage, and strength and protects us from various problems hence it is very important to connect with Isht Devta/Devi.
Our forefathers had chosen appropriate Kuldevta or Kuldevi of the deity of family and initiated the practice of worshipping them. The purpose was to ensure that the spiritual powers of these rishis should help us live positively and remove negative influences from our lives. This was to ensure that people could carry on with their lives positively and make progress on various fronts.
Actually, the respective Kuldevi or Kuldevta is the protector of the family. They are the basic authority in matters related to family and they should be worshipped first and foremost. Any puja is incomplete without first invoking the Kuldevta/Kuldevi.