They're not practical, but honestly a lot of 40k is like that. What about all the chaos marines with huge spikes and horns on their armor? What about fat plague marines? Eldar and Dark Eldar both have stupidly tall helmets. Commissar have preposterous hats that ought to catch on doorways all the time. How do terminators fit through doors when they're so damn wide? They just do. I like the crests on their helmets just because there's a wide variety of them, so it helps rubricae look rather distinct from one another when you assumable them. It's a lot nicer than a whole tac squad wearing the same Mk of armor. Plus, it looks cool and adds to the whole Egyptian aesthetic.