There are two main attributes for a subscription billing software:
The primary (and the obvious) attribute:
Enabling and streamlining the billing process for you, and thereby giving you the ability to realize the fruit of your labor - the labor of acquiring and serving your customers.
In less dramatic words - collecting payments from your customers, through a flexible range of payment options (credit cards, debit cards, PayPal, Amazon Payments, ACH, wire transfer, checks, cash, etc.).
The not-so-obvious, but equally significant attribute:
Taking care of the operational side of subscription management and billing (adding credits, extending the trial period, changing the billing date, offering a discount, activating/cancelling a subscription, etc.).
These are the tasks that are generally carried out by your sales/ marketing/customer support team, and are pivotal in managing your customer relationships.
A subscription billing software works on top of a payment gateway, and makes managing everything related to your pricing, subscriptions, payments, metrics, and customers a cake walk, so that all you have to do is just sit back and focus on your product.