Multi-channels DMA Subsystem for PCI Express
Well, just as an example, I have a card that processes HD video. It allows the user to take single-frame "snapshots" of the uncompressed video. It also scales the video to three different resolutions and compresses them (H.264). Each of these four streams of data needs to be transferred to a different area of host memory, so there's a multichannel DMA controller to handle that. It makes sense since the streams operate at very different data rates, and in the case of snapshots, very intermittently.

Trying to use a single-channel DMA controller to handle all of this would be very difficult to get right and needlessly complicate the software driver.