I do not understand when a custom dtd is needed

System

I was following the manual examples for FormAPI and creating a custom dtd for each DCT that uses FormAPI. But then I realized that I forgot to create a custom dtd for one DCT and it still works perfectly. So, now I am confused as to when they are needed. I don't want to create work for myself. So if they're not needed, I don't want to waste my time. Anyone have some insight on this?