PO(purchase order) & Movement journal related to the SO(sales order)

Hello, 

I have 2 important questions and i need brief explanation:-

1st: why we need to use inventory movement journal to adjustment inventory instead of purchase and sales order ax 2012?.

2nd: what is the diffrent between the PO(purchase order)  & Movement journal related to the SO(sales order)? .

just need good explain to take the idea for the two questions

BEST REGARDES