It's a lint filter, and the problem isn't just with septic systems. The discharged water is full of fibers. Much of it takes forever to break down if it does so at all. But even before it reaches your septic system, the fibers can build up in the pipes and clog them. I learned of the problem by needing our drain pipes professionally cleared because of the washing machine dischage.
I stopped using the in-wall drain. Even if you use a filter, you have to remember to regularly clean it or water will back up, and the mesh filters still let much of it through. If you have a nearby utility sink, discharge into that, instead. Fasten an old stocking around the end of the discharge pipe. It will capture virtually all of the lint. The stocking will balloon when the water discharges, providing plenty of surface area for the discharge, even when it has a lot of captured lint. When the stocking gets too full of lint, toss it and stick another one on the hose.
BTW, the in-wall outlet has a trap that will eventually dry out if you aren't regularly discharging water into it. If you don't use it, pour a quart of water into it to ensure the trap is full, and then seal it to stop evaporation and keep out funky-smelling sewer gases. You might find a rubber stopper that size. I used some rope caulk and a small plastic lid a little larger than the opening.