These crests probably were used exactly for "advertising" during the mating season. Some birds have strange crests too although much smaller in size. Anyway, they must have some purpose even if we can't understand what.
Oh certainly it was for advertising but the cost to the individual would have been very high unless it was some sort of super rudder enabling hairpin turns in mid air (but I doubt). I thought of toucans and hornbills but these are specialised niche birds and their bills tie them up with big costs. They are both long lived and slow reproducers too.
It's very nice, I like the way you can just look through the clouds to see a small piece of land. The clouds do seem quite 'solid' though, as if they're not transparent enough. (not that I could have done it any better) Are they a pair? (male and female)