As FPGAs increase in size, the data sources and destinations tend to grow as well. While it is possible to add a new DMA for each data source and destination, it is often sufficient (from a bandwidth perspective) to only have one DMA and to channelize the data. This saves resources in the FPGA, while still being able to identify the data from software and hardware. The resource savings improves as data widths increase as well; which is also a trend.
Another benefit from additional channels is the ability to prioritize the data coming to the DMA. The prioritization of channels is highlighted in the AXI MCDMA PG288 .