That means that light penetrates deeper into Neptune's atmosphere. So the clouds we see at this wavelength are deeper than the ones we see in the 889-nm FQCH4P15 filter. This allows us to study the vertical structure of the clouds, or how they vary with height. If there is time, I would also add the F410M filter, since this gives an even better handle on the contrast of any Great Dark Spot further out in the blue. These exposures are so long that cosmic rays are a significant problem.
Cosmic rays are high-energy particles that pervade outer space.
By taking two pictures, we can decide what is real cloud structure and what is just a cosmic ray hit. This is less of a problem for the other wavelengths, since the planet is a lot bright at those wavelengths, and there are a lot less clouds with high contrast. It also takes a full minute to change filters. That adds two minutes minimum to each picture. It also takes time to rotate the filter wheel to the proper positions for the two methane quad filters, so that means another minute for each of those.