The same instruction, thousands of times a day.
The whole model, every time.
Sorting a support ticket into a queue. Pulling six fields out of an invoice. Deciding which of twelve intents a message is. Each of these reached production on a large general-purpose model because that was the quickest way to make it work. It still runs there, and every call pays for a model built to do far more than that.
A smaller model trained on your own examples can be good at one task. Whether it is good enough at yours is a question with an answer. Getting that answer means building a dataset, an evaluation, a training run and a comparison before there is anything to show for it, so the question tends to stay open.
When fine-tuning is the right answer, and when it is not ↗