I don't really understand the benefits of Chargify versus using Authorize's Automated Recurring Billing. Authorize's ARB is only $20 a month, no matter how many

I don't really understand the benefits of Chargify versus using Authorize's Automated Recurring Billing. Authorize's ARB is only $20 a month, no matter how many customers you have. If one needs an Authorize account to use Chargify why wouldn't I just use Authorize's solution?