Processor usage. With DMA the processor doesn't have to do anything - the data will be stored automatically. Without DMA the processor needs to get the ADC data and put it somewhere or do something with it.
DMA is direct data transfor,general I/O data transfer flow is CPU read a byte each time,but when the data is huge,this action will waste much time. So the DMA controller tell CPU I have data to handle,you don't disturb me. When I transfer finish,I will tell you through interrupt.