DMA mem<>mem vs device<>mem

Hello,

I am not sure I understand the difference between dma mam<>mem to mem<>device.
I am writing ethernet driver which use dma between ipstack buffer to
fpga buffer (the fpga implements in hw the the ethernet mac).

When moving buffers from one address (ipstack) to another (fpga) isn;t
it just memory to memory ? Why is the need for memort to device ?

Regards,
Ran