Normally, a low pass filter is used to reduce the effects of moiré and aliasing. The F5 and F55, however, don't suffer as much from these effects due to their much smaller pixel pitch. The main use for an optical low pass filter in this case is to soften the image, giving it a more "filmic" look - what Sony refers to as "softer, organic images."
The cameras use the sensor in three different manners: you can use the entire sensor and every pixel for 4K recording, the entire sensor for oversampled 2K and HD recording, and center-cropped for 2K and HD. Below are some examples of all three sensor modes with the 4K filter and with the 2K filter for comparison.
Edit: The 2K OLPF's main use is to improve the image quality of 2K High Frame Rate Recordings and 2K Full Frame Mode, but it also has the added benefit of providing a more filmic look to the 4K recorded images. This filter is not a required accessory, but it is helpful for certain highly detailed scenes.
4K Image Scan
2K Full Scan
2K Center Scan
