pinging back links

Hi! I want to find a detailed tutorial which explains why we must ping our links? What's the goal behind this? If you post a link do-follow/nofollow then google will find it so why? Thanks!

Hey guys, I like doing case studies and testing out products before I use them; I had some interesting results with one of my latest ones and thought I would share it with you guys. There is a lot of debate over crawling vs indexing-whether or not crawling is beneficial, and which tools...