What is the point of credits as a free user?

Hello! So I have not used ChatGPT since around GPT3. I returned using a referral and received credits to try it out (referral) however i downloaded the app, and I attempted to use both Sol or Astraea and it blocks it telling me its unavailable with a chatgpt account.. I honestly thought credits might have been an exception but I guess not? As a free user how are these used?