How would that differ from buying $20 worth of API credits each month?

1) security (oauth is much more secure than a static api key; if your key gets stolen, a hacker can run up your bill)

2) AFAIK the $20/month plan allows use of more tokens per month than if you bought $20 of tokens. my understanding is it assumes most users will only use a fraction of that each month, and they rake in profit (like a gym membership)