Any scenario that features tightly coupled and interlocking business processes between organizations is a candidate for an extranet. Usually, connectivity is achieved through the use of the IP protocol. Security at the boundary between two intranets or an intranet and an extranet is more of a problem. Between intranets, security depends on a certain amount of trust and mutual understanding. Security lapses on either side may cause a breach of the entire system.
Some organizations connect their intranets through private networks, at great cost.
But open protocols by their very nature are insecure. Open protocols allow larger, more robust, networks, but the larger a network and the more open the protocol the more vulnerable it is to mischief. This means that the same physical path is not maintained for the entire communication session. Individual packets from a single message may be scattered across the continent before arriving at their destination. There is much that can go wrong, accidentally and deliberately, during this process.