Never having seen cassowaries in the wild, I don't know how accurate the old chesnut about them using their crests as 'foliage deflectors' really is. They are known to occasionally use their crests as tools, turning over leaf litter and soil - we have covered this on the list before.
Hornbill crests may act as resonating devices that amplify their calls - this was argued for in a paper from the early 90's (_J. Zool._) but I don't know if it's ever been tested.
For a detailed discussion see Kemp, Alan, 1995, The Hornbills (Oxford UP), pp. 19 and 22. Kemp notes that in most species the casque contains little or no bone, and may have first arisen as a supporting structure for the bill as it is no more than a raised ridge in some species. The casque usually contains an uninterrupted air space, with only a few bony struts at the rear. Kemp notes that experiments on fibreglass casts have shown a correlation between call structure and casque structure, suppotring the acoustic-amplification hypothesis. The exceptional species is the Helmeted Hornbill Buceros (Rhinoplax) vigil, in which the front end of the casque is a solid block of "ivory", such that the skull accounts for 11% of body weight. The males use this in fighting - they will sometimes butt casques in flight (!) for as long as 2 hours - but it is also apparently used to make the bill into a weighted chisel for hammering the bark off trees.