Why We are Doing Accrual and Deferral in SAP?

Accrual refers to Income/Expenses earned/incurred but have not been paid, while in deferral the income/expenses are paid in advance for the services given or used in a period of time ahead, the same concept is applied in sap and the amount that is actually to be paid or received is debited/credit in the current month and the rest amount is reversed next day, the concept of running this program is same as on a day of the month 1500 debited on A+1 day 1000 reversed, 500 utilized as accrual.

Thanks for your comment. I understand your explanation but can you please give me a example so that I can better understand.