으니까요

Because, added on politely

It supplies the reason for what you just said, tacked on as its own polite sentence: because you can buy everything in one place, because a lot of people have rain boots, because people will not all react the same way. Putting the reason last like this turns it into an explanation rather than an argument, which is why it sounds gentle. It is the ordinary because-marker with the polite 요 on the end.

Register: polite conversation