Sonic resells AT&T fiber when their own fiber offering is not built out in that area. This allows the customer to utilize Sonic's services and support (being able to call Sonic instead of AT&T is a feature for many people) as well as use sonic services, and allows Sonic to still make a small amount of money per customer and have a built in audience of people what likely want to upgrade if Sonic fiber is built in the area as it will be cheaper for them (and still more profitable for Sonic).

Disclosure: I work for sonic, and this is my personal understanding of the strategy, which may have other nuances.