Some providers offer better pricing if you use their modem/router combo. For example, Xfinity plans come with a 1.2TB monthly data cap. So, you might have the following options:
A) $50/month with your own equipment and a data cap
B) $80/month for the same as (A), but unlimited data
C) $60/month for the same as (B), but higher bandwidth and using the ISP's equipment