For the chat models, they're all finetuned slightly differently in their prompt format - see Llama's. So having a conversion between the OAI api that everyone's used to now and the slightly inscrutable formats of models like Llama is very helpful - though much like langchain and its hardcoded prompts everywhere, there's probably some subjectivity and you may be rewarded by formatting prompts directly.
The slight incompatibilities of prompt formats and style is a nuisance. I have just been looking an Mistral’s prompt design documentation and I now feel like I have underutilized mistral-7B and mixtral-8-7b https://docs.mistral.ai/guides/prompting-capabilities/
