I'm not exactly sure either. If you have enough bio balls to support the amount of bacteria for your aquarium the added surface area would be no added value at all. From what people that like scrubbies say they have more surface area. I guess maybe if they didn't have room for an optimal sized sump that would support their tank with bio balls they could use scrubbies instead. If you're just using scrubbies, because it's your personal preference with no real world added benefit that's understandable. The wet dry I just built for a 75 gallon aquarium holds 7 gallons of bio balls which is way more than necessary. I could've used scrubbies, but the added surface area would've been pointless.
I don't understand why people are saying to use a good prefilter and that takes care of the stuff building up in the scrubbies. On one of my tanks I use a 25 micron filter sock and it lets some stuff through that would be slowly building up in the scrubbies. In another tank I use a 100 micron filter sock and it letd a lot of stuff through it. I actually put 3 shrimp in that sump to try and help keep it cleaner.
I don't understand why people are saying to use a good prefilter and that takes care of the stuff building up in the scrubbies. On one of my tanks I use a 25 micron filter sock and it lets some stuff through that would be slowly building up in the scrubbies. In another tank I use a 100 micron filter sock and it letd a lot of stuff through it. I actually put 3 shrimp in that sump to try and help keep it cleaner.