You chose frangible roofs when the chance of over pressure is very low (fire case usually) and you can allow the roof to "flip" up and relieve the pressure as the least worst thing to prevent catastrophic failure of the tank shell itself.
For many fixed roof tanks this applies so it is fairly common, but far from "compulsory".
I hope you're just asking out of interest and not because you're trying to design or specify a tank??
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
