persistencebroker

when i thought everything was in the right way.. 
persistencebroker class turned up to me.. i feel i'm lost.

I can't understand how persistensbroker works. 
Why save, delete, update, etc. methods appears again, there are 
those methods in several persistence classes and all do the same 
think, i thought this redundant(if doesn't can you tell me why?).

Why all existing classmap objects must be loaded into memory 
when persistencebroker is created?, i thought a classmap 
instance must be created at a time when it matching bo was 
created, i mean if student object is created then its own 
classmap(with attribs., cols., etc) instance will be creatad 
throught some persistenceobject method and linked to that 
bussinesobject, but not that all classmap instances in the app. were created
before any bo instance were created