A couple more points to address some of the other questions:
From a billing perspective ChatGPT and the API are handled completely separately. There is no funds transfer.
There’s quite extensive documentation on the API developer platform including relevant code snippets. Additionally, you can find a lot of worked examples in the OpenAI cookbook.
It just really provides you with a lot more flexibility including (but not limited) to the possibility to adjust specific hyperparameters. How relevant that is to you, really depends on your use case(s).
I’d echo what others have said. You can’t lose out by just trying it out in the free tier. That will provide you with access to GPT 3.5 and a selection of the other models (e.g. dall-e, text to speech), the playground etc. and will give you a taste of the differences relative to the regular ChatGPT experience.