Our billing runs daily and charges each customer once every 30 days based on when they originally signed up for a paying account.

So each night, we bill about 1/30th of our customers. It's not exactly 1/30th because some days have more customers and some have fewer.

Some companies do monthly billing runs and then pro-rate new customers (Linode, for example). Others do daily billing runs and charge customers on the anniversary of their signup. The latter can be more challenging in terms of accounting and technology in some cases but has the psychological benefit of money "streaming in" daily.

I interpreted it as each subscriber can be billed on any day of the month as opposed to everyone billed on the first for example. This would help the cash flow.

Cash flow is definitely a big part of it. With daily billing, we never see any huge fluctuations in our account. In my opinion, a slow and steady stream of income is much better than big pop once a month.