Based on code licensed from Mentat's Portable Streams product, Open Transport was built to provide the classic Mac OS with a modern TCP/IP implementation, replacing MacTCP.
[1] Prior to the release of Open Transport, the classic Mac OS used a variety of stand-alone INITs to provide networking functionality.
STREAMS had a number of advantages over sockets, including the ability to support multiple networking stacks at the same time, the ability to plug in modules into the middle of existing stacks to provide simple mechanisms for filtering and similar duties, while offering a single application programming interface to the user programs.
Additionally, most Unix code still used sockets, not STREAMS, and so MacTCP offered real advantages in terms of porting software to the Mac.
The vaunted flexibility of the Open Transport architecture was undermined and ultimately made obsolete by the rapid rise of TCP/IP networking during the mid-90s.
Open Transport was abandoned during the move to OS X, which, being derived from BSD, had a networking stack based entirely on sockets.