My GPT vs OpenAI API, what's the difference?

Dear community, please ELI5: If I can build my own GPT on OpenAI platform, why would I use OpenAI API and tokens? What is the difference? Just to mention, I am on a pro plan, playing around with my GPT and it can process very large texts (over 200k words) in seconds.

This will let you customize the behavior and training of your GPT, but using OpenAI API gives you access to pre-trained, highly-optimized models without hosting or maintaining them in-house. The API also offers scalability, reliability, and updates. Your choice depends on control vs. convenience.