Why do we use payment gateway?

We are using payment gateway in our products to get digital payments. Payment Gateway will be used in following applications:

AR Store - AR Store is a online shopping application where user can purchase items and pay digitally using paayment gateway.

Housing Society Club - Housing Society Club is the application for housing societies and their residents. Using payment gateway, Residents can pay their society maintenance to the society. Here we (Khandelwal Software Services LLP) collects the payment from residents using payment gateway and pay it to the related society account in 3 to 5 busisness days.