When, why ,where are these models used and in what circumstances ?
a:-Usually in service based company this model is used as people who work will not be there permanent so they need lot of documentation who,what they do in project so that later people who replace should understand.
b:- For each stage they need lots of approval and concurrence from SA , Higher exp person for any changes in system or software. Lot of developers and people needs to move in and out on regular basis .
Spiral Model:- This is a beautiful model which tells risk of all phases and will give small model output every stage of LifeCycle.
RAD:- This is usual practice by young and enthusiastic developers who try new things in a rapid way who is least worried of documentation and give output soon.
When project comes SA or ITA or those who have higher view of the system they understand the severity and impact of the change in whole system based on that they decide which model needs to be applied with best result. But all organisation will follow some standards SDLC but based on problem / project its modified internally and make sure all are in company CMMI standards .