Charlotte Blair
Ansys Employee

There can be many reasons why you have a signal and ground trace on the same layer of a board or chip. These layout are usually described as co-planar type of geometries. The need for co-planar configurations may be the need for direct access to ground, to avoid any parasitic. The ground layer on the board may be electrically long to attach a via ground. There is a power layer between the signal and ground.

Hope this helped!