#Nandi, also called Nandikeshvara and Nandishvara, is the name of the gate keeper of Kailasa, the abode of Shiva..
#Nandi got the Divine knowledge of Agamic and Tantric wisdom taught by Shiva from goddess Parvati. He Could teach that Divine knowledge to his Eight disciples who are identified as the progenitors of
Nandinatha Sampradaya -
Sanaka, Sanatana, Sanandana, Sanatkumara, Tirumular, Vyagrapada, Patanjali and Sivayoga Muni. These eight disciples are directed to eight directions of the world by Nandinatha to spread the wisdom he taught them...
Why is Nandi Worshipped
with Mahadev
Nandi is Shiva's animal form 
He is also the Guardian deity of Kailash, the abode of Shiva..
Nandi is a medium to reach "Mahadev"
Nandi(one who brings happiness) the biggest devotee of Mahadev is the son of Shilada. Shilada wanted a Son like Mahadev or having qualities like him, he performed severe penanace for it. Mahadev , our Bholenaath , Aashutosh agreed...
During Daksha Yagna , Daksha spoke rudely of hashtag#Mahadev and Nandi closed his two ears, chanting "Siva , Siva". Its believed that whispering your prayers in the Nandi's ears will reach Shiva directly. As a faithful attendant Nandi informs his master of the same.
While they were meditating Jalandhar who was an enemy of Shiva abducted Maa Parvati. Shiva was not aware of this continued meditating. The demigods did not know how to communicate about this incidence to Shiva.
As #Mahadev sits in meditation, Nandi makes sure that he is connected with the Mahadev...He sits as his guardian. It's only Nandi who communicates messages to Shiva during his meditation so that he can come out of his meditation...
Nandi is described as the son of the sage Shilada. Shilada underwent severe penance to have a boon– a child with immortality and blessings of Shiva, and received Nandi as his son...
Nandi is considered to be chief among the Siddhars, initiated by Parvati and Shiva. He is regarded to have passed on what he had learned to his eight disciples, namely, the Four Kumaras, Tirumular, Vyagrapada (also known as Pullipani),Patanjali, and Sivayoga Muni..
They were sent out in eight different directions to spread his wisdom. His teachings are held by the Nandinatha Sampradaya, a line of gurus who claim descent from Nandi...
" ॐ नमः शिवाय "
