The reason is that once you know the codes, you can navigate and enter data at a genuinely incredible rate. If you've ever watched an experienced operator on one of these applications you'd be amazed. For one loan processing application, the terminal was an order of magnitude faster than the Web-based interface that was being deployed.
The key here though, is "experienced". This is someone who has developed the knowledge and muscle-memory to operate at that speed. This also makes the system more brittle (and often harder to change). Critically, it also means it takes a lot of training and time for people to be productive.
The reason many big organizations dropped the terminals - they wanted to be able to train people quicker, not because it was more productive.