Redundancy as you noted. Also dedicated lines directing flow to different areas of the display. If DC pump can alternate power level (I've done left side, right side, alternate power to simulate high, low, tide).
With regards to power failure it does matter. You can plug them into different controller power bars. You can plug them into different power panel circuit breakers (ideal). Can also leave one on, one off, in case of failure or maintenance turn one off, stop code/programs, alerts, swap them out, enable code, clean, replace.
There are a lot of different reasons why people run two return pumps. They are a pretty vital part of our display life support. I run two with a dry spare. I never clean Friday - Sunday in case something happens. All pumps are the same so I can swap as needed or use one for a can bird.