To control Nitrate and Phosphate. If you have high nitrate and high phosphate, and you can’t get them down, refugium is one option
Nutrient export is just pruning the algae. by removing it you take out those nutrients and you free up room for more growth/ nutrient exports
Yes. That’s the idea of the Refugio is give the algae a place to grow there instead of in your display
Everything is pretty much here: https://www.algaebarn.com/blog/refugiums/
Maybe I can jump in and share how nutrient export works in my tank. I have a 56 gallon display with a 30 gallon sump/refugium. Generally refugiums have some rock and macroalgae. Some have sand as well. Both my display and refugium have a lot of macroalgae. The macroalgae grows and they use the nutrients to do that, kind of like fertilizer. Then when you trim or prune the macroalgae and remove it from the tank, your are actually removing nutrients. I choose coral that like nutrient rich water, like soft coral and some LPS, plus a beautiful purple photosynthetic sponge. In the display the removing/pruning process is really important or the tank will be overrun by macros. If the macroalgae is confined to a refugium one can prune a little more casually and not worry so much about how the tank looks. This is the refugium and display.
