Creating Memory device for Nios II processor

Is there a specific reason for using the DMA controller instead of declaring the interface as a RAM module? The data I want to access is stored in on-chip memory. In SOPC builder / Qsys there is a on-chip memory available so this should be pretty similar. The most obvious difference for me would be that I would use one port of the dual port to interface with my custom logic and the other port would be used to interface with the Nios II processor.