Paying monthly for a product/service tends to make that product or service used more, because people feel the need to justify seeing that item on their bill.
This is why, for instance, so many gyms moved away from 'buy a year' plans. People would sign up in Jan / Feb / March, go a few times, and never go again. When it came time to 're-up', they wouldn't because 'we never go to the gym'. However, when people were billed monthly, the would go maybe even only once or twice a month, but it kept them using the service and the 'hurdle' for payment each time is much lower.
Lest you think it only apply to gyms, the study also looked at cell usage when the only thing billed per month was overages, and you paid the full contract up front. They saw a similar type of usage, where even with something as common as a cell, the annual cost was higher and the average monthly use was lower.
Bill monthly (or even 'micro' -- 'on use') and you make more money overall.
I don't want to worry about monthly bills.