Don't let the name fool you — acasual kilt is simply made with less fabric.
A standard gent's kilt is made from 8 - 9 yards of wool, and a casual gent's kilt is about 5 yards. Both look very similar (the main difference is the depth of the pleats), both are made of 100% pure new wool woven in Scotland, and bothwould be appropriate attire for a formal event.
Less fabric means that the casual kilt is lighter weight and might be better for warmer climates.