Many ancient Shiva Temples could be witnessed in Kashi. one of them is the Brihaspateeshwar Temple. The importance of this Temple is mentioned in Kashi Khand.
Guru or Brihaspati was Angira’s son. He was acting in a gentlemanly, leadership, and good Samaritan manner, showing respect for authority figures like instructors and elders, among other positive traits. He arrived at Kashi, built a Shiv Ling, and for several years continuously offered devotion to Lord Shiva. Through that Ling and in the shape of Jyoti, Lord Shiva came to him (Divine Light).
When Brihaspati first saw Lord Shiva, he was overjoyed and immediately began singing praises to him. The Lord then awarded Brihaspati the right to rule over all Devas. Lord Shiva approved Brihaspati’s request to be given the name Vachaspati since he heard him singing his prayers (having excellence in speech). Brihaspati will use the name Brihaspateeshwar for the Ling he installs.
Lord Shiva said that a devotee who regularly conducts pooja, etc. to Brihaspateeshwar will have all of his requests granted. Then, Lord Shiva commanded the Devas Brahma, Indra, and others to appear. Lord Shiva decreed that Vachaspati (Brihaspati) shall serve as the Guru (Teacher) of all Devas in the presence of all those celestials. Vachaspati will ascend to the position of lord over all wise people. When the Pushyami Star conjoins on Thursday, a devotee who offers prayers at Brihaspateeshwar will succeed in all of his endeavors (Pushyami star is called Poosam in Tamil and Pooyam in Malayalam). A person gets absolved of all his sins if he prays to Brihaspateeshwar for six months in a row. (Chapter 17 of Kashi Khand)