1) CISCO's well documented efforts to intentinally put them into place in the canonical sense of the word (for LI): https://news.ycombinator.com/item?id=22251965
2) it is a well-known feature that is used to change the administrative distance of eBGP in order for an interior gateway routing protocol (IGP) to take precedence over an eBGP route. https://community.cisco.com/t5/networking-documents/what-is-...