For starters, the naming is much less confusing. But the behavior also appears to be enforced/validated at some layer (hopefully?), which function calling did not seem to be. I was experimenting with it a couple weeks ago and it would work like 75% of the time but would often give me invalid results for schemas with relatively simple nested objects.

This post (from an OpenAI researcher) contains a bit more background: https://news.ycombinator.com/item?id=41174213

This is guaranteed, function calling without it is not. The old way can work for you, but my experience is different, especially with complex schemas.