OpenAI API has a larger surface area than just completions. You can retrieve files uploaded through it, you can generate images, you can use the embedding API,

OpenAI API has a larger surface area than just completions. You can retrieve files uploaded through it, you can generate images, you can use the embedding API, you can use a different, more expensive model (GPT4) - and make as many calls as you wish. With a backend, you can restrict what is allowed, add your own rate limiting, you can rotate keys etc.