Hi Rajesh,
If the company enter the transaction on daily basis then there will no different on those dates.
Mostly both dates will help for backdated entries.
Below is some example,
Scenario for outgoing payment,
A finance person giving advance 1000rps to some of his employee, bcz of his lazy he has not entered in the system or system problem he has not entered, so he is giving cash with manual receipt.
Next day , if he want to enter the entry in the system, he supposed to enter Posting date as current date and Document date as previous date.
Scenario for Sales Order/Quotation,
You are getting order confirmation mail on leave days, now you supposed to enter the Sales order Posting date as current date and Document date as on mail you received.
Mostly this will help you on quotation, for example if you receive quotation on mail you can just reply in same mail, but you can enter it in system later.