What is Abhishek?
Abhishek means to pour healthy things over the Lord, we pray. It is an easy way to please your Lord.
Rudrabhishek is to pour healthy things over Shivlinga and at the same time reciting mantras from specified scriptures. Puja is done not only for Lord Shiva but for Shiva parivar that included Maa Parvati or Gauri, Ganesh ji, Kartikey and Nandi Bail.
Why Redrabhishek is done:
It is done to get blessings of Lord for various purposes. Like
1) To get money
2) To get job
3) To get rid of your opponents in office
4) To get progeny
5) To get rid of diseases
6) To overpowering opponents
7) For name and fame
Different materials are used to do this. Like for getting money, Abhishek is done with sugarcane juice; for progeny, it is done with milk and ghee. To get rid of diseases, Puja is done with water and special kind of grass. For overpowering your opponents, sarson tel is used. And for name and fame, panchamrit is used.