Some cephalopods are able to fly through the air for distances of up to 50 metres (160 ft). While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to be expelled from the funnel while the organism is in the air. The animals spread their fins and tentacles to form wings and actively control lift force with body posture. One species, Todarodes pacificus, has been observed spreading tentacles in a flat fan shape with a mucus film between the individual tentacles, while another, Sepioteuthis sepioidea, has been observed putting the tentacles in a circular arrangement.
Flying squids are cool. A lot like flying squirrels - more like gliding than proper flight.
My guesses are similar to the ones in the video mozz linked: developed for migration, or avoiding predators (including intra-species), or perhaps both. Air travel might be difficult due to gravity, but it’s a pretty handy tool if you’re trying to survive.