Having ChatGPT integrated into iOS 18 will be a big deal for iPhone users. Apple's language models can't currently offer chatbot functionality on par with OpenAI, Google, Anthropic, and others. If they could, Apple would simply send those complex Siri prompts to its own "AppleGPT" rather than partnering with other AI service providers.
The partnership is also a big deal for OpenAI. The company has a unique chance to score a big victory against Google and other rivals by getting prime placement on the iPhone.