sales order and the movement journal inventory transaction

Very Important quastion need to explain? 

1st *why* we use (inventory movement journal transaction instead of  adjustment inventory) for *what purpose* ?...

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answer plz..

 2nd why while creating the purchase order or even sales order we are not to using them to put the quantity and make all the processes from one place 

for ex: when we create or doing the SO entry (from confirm to post invoice)  like that the end of the entry right? 

but why we need to go to put the quantity it self from the movement journal and posted to finish the SO process and after that the last step we do the post invoice 

 and i want mention exectly my compiny are use the movement journal form that is why my question related to this point only

thanks