I think the main benefit of instruction finetuning was that the model is less likely to get prompt injections and more likely to follow the specified format?
Pure davinci was easily confused by what was system prompt, user prompt and its own answers.
To be fair, there are still many jailbreaks for instruction tuned models. Surely less than before, but it could be because models got smarter (Claude often "reasons" that something is fishy even if it appears in the prompt or appears like a system prompt).
Pure davinci was easily confused by what was system prompt, user prompt and its own answers.