Most importantly because you have a failsafe in the event that one filter stops working.
Increased control of water currents and therefore fewer dead spots where gunk can accumulate. Plus being able to have two intakes means that there are now two instead of only one part of the tank where dirt will be pulled into the filtration system.
More surface area for mechanical and biological filtration. More space for specialized media. This is especially important for more advanced systems. For example my two blackwater systems have one canister filter used almost solely for peat filtration. They have only pre-filter media, a medium pad, and then a whole lot of peat.
Easier maintenance since the increased surface area allows you to go longer between cleanings. Two filters will take longer to back up or clog than one filter.
Rotated cleaning schedule guarantees you can never make the mistake of obliterating all of your bacteria in one fell chlorinated swoop. It also means that there is never a stop to the filtration flow even when you're doing your weekly maintenance. A filter is always running.
Plus there is a fun sort of side to it - you get to use different filters. I rather like having both a Marineland C and an Eheim Ecco on my 30g tank. I like the variety. I guess it's part consumerist and part desire for experiential knowledge of products.
Why???
