Well from your question I assume you are layout engineer. Therefore I also assume you should have some EE degree and then you should know what those devices are.
In your own interest would be to understand crossection of all the devices you will be drawing. Also try to learn something about matching. It will help you in your job if you decide to keep it and be good at it.
To answer your question - you need them integrated or the final size of your iphone would be about the size of suitcase. Same way you would be able to ask why we integrate transistors. Not all blocks in "purely" digital circuit are digital....
PLLs oscillators, references etc.