Why would I use the FLIP button to rotate the image 180?

While the Context image is oriented as close to the common "north-up" convention as possible, the available Browse Format images do not necessarily follow this convention. In these cases, the FLIP button allows users to match the Browse image orientation to the Context image orientation.
Browse Formats that are presented "as-collected" during the ascending orbit node may benefit from a FLIP; these include IR-Browse and BPOL browse (see FAQ about North in Polar images). Similarly, RGB and DCS browse that are generated from polar projected images may need a FLIP to match the Context image.

An additional use of the FLIP button is to reverse the effects of the sunlight direction optical illusion in some images. Most likely, your eyes have become accustomed to seeing topographic features in THEMIS-VIS with sunlight coming into the image from the left side. Later in the mission, the Odyssey orbit changed such that the sunlight is coming into the image from the right side. This can create the optical illusion that craters and channels (topgraphic lows) are domes and ridges (topographic highs), and vice versa; use the FLIP button to reverse the effect and correctly "see" the features.