The short answer is that you do not have to have it. Some people prefer a flat ridge because it gives a nice smooth outline to the conservatory roof and has a more 'modern' look to it.
There are a few practical reasons for having cresting on the roof though, but I am not entirely convinced . It is said that birds will not land on the ridge if cresting is used, and subsequently will not poo all over your nice self cleaning glass roof. I am not too sure who came up with this theory because my neighbours' roof seems to attract starlings better than my three bird feeders.
So it would seem that the jury is still out on this one!