To be honest I don't know that much about Nandi, except that he is always there wherever Shiv is. I was interested to hear he also goes back to the Indus Valley, as does Shiv, at least in image. Very little is know about the Indus Valley civilisation which lasted for 2000 years from about 4000 BC - they disappeared, but it is known they have yet to find and weapons from that time - something for you to ponder zenandnow.
Nandi is adored, as a representative of Shiv himself, as he has become totally devoted, and sits forever at Shiv's front door, always looking on Him. His mood tells all, and is the way all Shavites strive to be.
Shiv, or Shiva, or Rudra, or Mahakala, or many other names is the most mysterious figure. I don't know of another God anywhere in the world like Him. His name means 'auspicious', but really why Indians worship him is also mysterious. Mostly Indians pray to the Gods for the same things everyone else in the world pray to their Gods - for children, business, healing etc. But Shiv is not really one to pray to for such things.
Shiv is the external representation of our innermost truth. He is the God most associated with the last phantom of appearance before Brahman Nirguna - the ultimate unknowable, beyond all form or attributes. Hindus who worship him, know this, so their worship is not for personal gain of any recognisable type. He is final and absolute enlightenment. Thus it is the Hindus have such a concept at the core of their religion ... they are ultimate agnostics.
I'll just add, that I don't feel Nandi is connected to the famous cult of the bull which exists in Europe, and esp mediterranean countries - that is a martial cult. Robert Graves has posited the view that Gods are best seen connected to each other across different cultures, not by their name or form, but in the way they are worshiped - I think that applies to Nandi. There is nothing martial about him, yet there is the deep strength of the bull, but in repose and devotion.