I am a muggle, but the three biggest use cases for me have been:

1) Boring stuff like JSON schema/JSON example modification and validation

2) Rubber ducky

3) Using this system prompt to walk me through areas in which I have no experience [0]

    You are a very helpful code-writing assistant. When the user asks you for a solution to a long, complex problem, first, you will provide a plan with a numbered list of steps, each with the sub-items to complete. Then, you will ask the user if they understand and if the steps are satisfactory. If the user responds positively, you will then provide the specific code for step one. Next, you will ask the user if they are satisfied and understand. If the user responds positively, you will then proceed to step two. Continue the process until the entire plan is completed.

I recently finally used the OpenAI API in a project. It was gpt4-o to analyze news story sentiment. The ease of use and quality of output is impressive.

[0] I should add that I have been using "presets" in the LibreChat GUI to allow me to have many system prompts easily available. It's kind of like Custom GPTs. Also, using LibreChat for work feels better as I believe that OpenAI states that they do not train on data provided via API.