The usual logic for such tall low-gravity creatures is that we tend to assume taller is better, and low gravity removes some very real shackles which make tallness unstable.
I think one of the most fundamental advantages of such tall creatures is the ability to put sensor apparatus such as eyes and ears and noses far from the center of gravity of the creature. It takes a lot of energy to move the center of gravity of a creature, and that movement is often a commitment that predators can identify and act on. The ability to move a small sensor ball a distance, observe what the world looks like from that vantage point, and then decide to move is quite valuable.
A related trope would be that low-gravity worlders would move slowly and gracefully. This is highly related to the extended size of the creatures. Neuron transmissions are slow. They top out around 120m/s. The bigger one is, the slower the reactions must be unless one distributes those reactions (similar to how our patellar reflex, aka the "knee jerk," is handled in the spinal column, rather than the brain). However, this trope may have some truth to it. In lower gravities, sudden movements can be more dangerous. You can shift your momentum in ways that are hard to arrest. Thus, slow graceful movements may be valid.
(This forms a tail-chasing loop in movies. One easy way to make creatures feel slow and graceful is to make them very tall, because it is very hard to walk in a non-graceful way if you're tall in 1g, and most filming is done in 1g).
However, if we consider fast movement for a bit, there may be an advantage to long appendiges. The further your mass is from your center, the higher your moment of inertia is (a measure of how much you oppose changes in rotation). Control over moments of inertia is key to maintaing stability in high speed low gravity environments. A spine, in particularly, is very good at controlling moments to cause remarkable movements. Nowhere is this more obvious than a cat turning itself right-side-up when it falls. Cats do some really clever tricks with their spine to allow them to control their rotation to a remarkable degree. A creature with long appendages, such as arms and necks could do the same. In low-gravity combat, this could be a make or break skill.