It does seem to be the most convenient way to hold your helmet. Especially when posing for a photo. I am not an expert in photographic history but I believe the individuals being photographed turn of the century had to remain absolutely still for a fairly lengthy time, certainly by modern standards. Consequently, holding your haube by it’s spike or placing it on a table beside you would help.
Also Otto von Bismarck and Kaiser Wilhelm I. used to hold their Helmets on the Spikes
Source: wikipedia.org (Anton von Werner-Kaiserproklamation, zweite Fassung 1882)